I will start off by writing that your boyfriend is your boyfriend, and as Nikki J wrote, your son will always be your son and if you choose your boyfriend over him, you're the one looking at ending the family ties. Since I don’t know the ‘whole’ story, my two cents are based solely on what you have provided. It looks like you have more than one problem here. This is going to be a long post, so I hope you don’t give up on reading, please bear with me:
1) A motorcycle for an 18 year old is asking for trouble. Bikes are fast - really fast and you don't have the protection that you would with a car. I am an adult and a motorcycle rider. I took the MSF course and am licensed. I had my own bike for a couple of years and my husband had his, so if I wasn’t riding mine, I was on his. I took this course in my late 20's early 30's and decided to sell because it’s too darn hot in Texas to ride when you’re wearing, for lack of a better word, ‘protective clothing’ (ie: helmet, jacket, boots, gloves & jeans).
If driving a car while under the influence is hard - what does alcohol do to someone that is riding a motorcycle? If you don’t know the answer, it’s twice as hard because when a motorcycle driver is not under the influence they are already paying attention to ‘cagers’ (people in cars). When there is alcohol in your system all of your reflexes are basically gone! When driving a car, one has some type of protection (aside from seatbelts), one detail is that you don’t get thrown 20+ feet from a car at an intersection. People driving cars don't see bikes as well as they would another car – especially at night. Not only is this common sense, but it was discussed at the MSF course.
2)Your son is 18.. why would he be drinking anyway? Last time I checked 21 was the legal drinking age. Why would the police have asked you to come get him? He wasn’t at the legal drunken stage. Hmmmm, police usually take people in unless there was something else going on. Back to my point, I am aware that a lot of us drank at 18 and made some really dumb mistakes, but it is our duty as parents to try and educate our children – Drinking and Driving is not an option. You as a parent should be very upset that he was out drinking and driving. Your son knew the rule and that was the one he chose to break. I believe that cell phones are very popular now-a-days, why didn’t he call before getting on the bike? Why didn’t he take the responsible route and crash at a friend’s and pick his bike up in the morning? I would hope that he knows the difference between right and wrong.
3)I don't think your boyfriend is overreacting by taking the bike away. I’m thinking that he doesn't want the responsibility of your son being irresponsible, injured, killed or having him hurt someone else. I don’t agree with the ultimatum, but I absolutely, 100% agree with taking the bike away. Again, the ONLY rule was no drinking and driving and that was the rule he broke. Two or three weeks would be a great punishment for something completely different – driving ANY kind of vehicle under the influence is serious and indicates a lapse in judgment.
Nikki mentioned a GREAT idea and that would be to let the punishment be to sell the bike and buy a clunker with the money from the sale. I will add to have your son be party to these transactions so that he understands that there are consequences for the choices one makes. What would the consequence have been if he had hurt, possibly killed someone? OH and he wouldn’t have to deal with the outcome of killing himself, you would. I realize those last sentence was harsh, but this isn’t a game or a joke.
How can he ever prove that he isn’t going to do this again? Seriously. I realize that selling the bike may be extreme, but an 18 year old doesn’t tend to think clearly. If your son wants a motorcycle then he could purchase one on his own; he is an adult. If he has to wait to purchase one, well, that’ll be his goal and he’ll probably more responsible by the time he gets it. He could drive the ‘clunker’ until he’s able to purchase one on his own and then he’ll probably cherish it more.
4) Of course your son absolutely hates your boyfriend; he took away the GIFT that the two of you gave him, his precious motorcycle. All his friends thought that having one was really cool. He is probably more furious with himself than he is with your boyfriend. He’s going to lash out, that’s what teenagers (and some adults) do, that’s how he feels better about what happened. That being said, he doesn’t have to like anyone right now, especially himself. He made a HUGE, serious mistake and you as his parent should be the one helping him see this. Have a serious talk with your son and let him know that he’s going to have to deal with the choice he made.
I realize this is long, but this will be the last point:
5) Sitting down with your boyfriend is CRUCIAL. Having him give you an ultimatum, especially when it comes to your son, is a card he should have never pulled. I will repeat, I realize that I was not privy to the whole conversation, and I’m only hearing your side of it, but this is your life and you’re the one asking for advice here. Think very closely about the words your boyfriend chose to share with you when it came down to your son. YOUR SON. I believe that you should sit down with your boyfriend and discuss the whole situation – calmly. Your son does need to learn a lesson and should have to pay the consequence, but having you pick between the two of them is absolutely crazy. If your boyfriend still believes that you have to choose, in my opinion, your decision has been made for you – really. Once you sever the ties with your son that will NEVER be forgotten and as the concerned mother that you are, imagine what it’ll be like if you have to choose any person over him. Now if your son is one to use his role, as your son, against you to get his way, that is a situation you will have to solve, without a boyfriend in the picture.
OH, you never mentioned how long your boyfriend has been in your son’s life or if they ever had a ‘relationship’ outside of the purchase of this motorcycle.
A couple of articles you may want to read, if you don't want to purchase the book: http://www.vitalsmarts.com/crucialskillsarchive.aspx