I'm a single mom that struggled with this decision. Almost the exact same situation. I gave my daughter my last name, and I am so thankful that I did. Just from paperwork/legal standpoint, it makes everything so much easier! Most of the time mother's are the ones doing the paperwork for school,sports, etc. Sounds like in this situation you will definitely be the more responsible one. Anyways, it is easier when faculty/staff/coaches, etc can familiarize your child with you. Less questoins to answer. Sounds like this dad may skip out, so why should your child have the last name of an absent parent? My daughter's father has been involved 2 out of 3 yrs of her life. He has gotten married now, so if my daughter had his last name, he could very well pass off his new wife as her bio-mother (we look a lot alike, except I am about 80 lbs thinner). I wouldn't put it past him to try that either. If my daughter had his last name, I would have to constantly convince people that I am her mother, not this other woman. Some of my reasoning too was that my daughter will get married some day so her name will change then too. So might be different for you since you are having a boy. I would strongly suggest using your last name, or at the least hyphenating. Good luck!
I found this posting on another question, thought you might be interested:
http://www.mamapedia.com/questions/13564579137628209153
The question is titled "Being a step mom"
I've been a stepmom of two who are now 27 & 24 since they were both in diapers. You have an important role in your stepdaughter's life. Mine now call me "mom" and we are very close.
The judge and family court do not care about you, though. (We were in court several times, and we learned the hard way that our family court system is not a solution to anything - but can cost extreme amounts of money, energy, and emotion. Do not expect it to use logic or "common sense" - not one bit.) I was expected to sit silently in the back of the courtroom, dressed conservatively, and looking "nice". That's all.
Here's a resource written by a woman that I have met and respect: http://becomingastepmom.wordpress.com/
As far as the difficulties with legal, medical, school things that others have mentioned. I didn't run into that. I had the same last name as my stepchildren and never admitted that I wasn't their "mother" and I signed them into school, out of the hospital, and wrote them permission to go on a trip to Mexico, for example, and no one ever questioned me.
Good luck!
The part that concerns me and would probably concern you is
"I wasn't their "mother" and I signed them into school, out of the hospital, and wrote them permission to go on a trip to Mexico, for example, and no one ever questioned me."
Again I am so thankful that my daughter has my last name, because step mom and father already tried to pull some stuff. Give your child your last name!