Hello, I had the same issue that you do. When I first had my son I lived at home with my parents and my son slept in the same room as me. He actually shared a room with me until he was 3 1/2. The big thing here is, since you are in the same room, he will see no problem with crawling into bed with you. My son will be 4 in a week and he still will crawl into bed with me even though he has his own room and loves it. When my son was your son's age he would wake up 3 or 4 times a night without fail. There wasn't much I could do about it except snuggle up with him and go back to sleep. Since I was going to college full time it was the only way I could get a full nights sleep.
Once I graduated however I turned the tables a bit. He had just turned 3 and had developed enough to the point of where he would only wake up once maybe twice a night. Since I usually woke up to his loud feet on the hardwood floor I would refuse him entry into my bed. I would give him a kiss, tell him I love him too, and point at his bed and tell him to get his fanny in it. As long as you are persistant he will eventually give up. (As a plus, if I ever fell asleep on my son doing this, he usually just went back to his own bed!)
I am also of the opinion that there is nothing wrong with children crawling into bed with mommy. Most of the time (like with my son) it was just a way to get closer and have mommy time while he was sleeping. During the winter it was usually to get warmer as well. When you stop and think about it, isn't it really nice when you get to snuggle up to someone special? To your son, you are that someone special.
Now my son just comes in at about 6 am for an early morning snuggle. Sometimes he falls back to sleep, other times after he finishes his cup of juice he wanders off downstairs to play. I only got it this way because I figured out the best time to put him to bed (his bed time has a tremendous effect on how many times and when he wakes up, believe it or not) and because I would insist on him sleeping in his own bed until a certain time. Obviously this time has leeway cause if he comes in at 530 I dont send him away and he's likely to fall back to sleep anyway.
A side note: I have NEVER believed in letting a child cry it out no matter what age they were. The younger they are, the more they need the assurance that you love them and will respond to them. So before putting him back into bed tell him you love him bunches, and check on him a few times so he knows you are paying attention and do care, it works very well with my son and I only ever need to check once before he goes back to sleep.
I posted a response similar to this one a short while ago on a mom who had problems getting her child out of her bed. Check it out and see if any responses there help. It takes time, a lot a patience and some missed sleep but in the end its worth it. Gook Luck!