The relationship is very new for everyone. Yes, there is a baby coming and that changes everything, but all of the normal parts of building a relationship aren't going to change their timeline just because a baby is on the way.
I would step back from the relationship - not necessarily end it right now (you'll be tied to this guy for years no matter what happens), but slow waaaaayyyy down. Your focus, as always, should remain on your son and on taking care of yourself so that you can take care of the baby you're growing. It's recommended that when a single parent starts dating, that the kids aren't introduced to the partner for many months, when you know the relationship has a chance of being solid. You don't want people coming in and out of your son's life.
Obviously you've introduced your son to this guy and his kids, but you can pause that. The relationship is so new that I'm assuming you were living with your son and supporting yourselves before this, so hopefully you still are doing that and haven't moved in with the guy. If you did move in together, stop that as soon as you can - have him move out, or you move back to your old place or whatever. Minimize the time you spend with your boyfriend - your time together should be adults-only, getting to know each other without the kids. Give yourself time to figure out what kind of person he really is and if he is the kind of partner you want around you and your precious son for years to come. I think in most relationship, true colors can take up to a year to show up - that said, it sounds like his true colors are showing already so I wouldn't invest a ton of time or effort in this if he is the jerk that you describe. If he doesn't measure up - is selfish, lazy, unkind, unstable, tons of baggage, etc. then it is what it is. Deal with him as a co-parent to the baby that's coming and that's it. Cut your losses and move on.
I've sort of been in your shoes. My oldest son was 5 when I got married to a guy who is now my ex-husband. We were friends for a year, dated for a year and engaged for a year so a lot of thought and care went into the decision. I did get pregnant a few months before our wedding so that was earlier than we had planned but not a disaster. He was sweet and fun with my son until we moved in together ahead of the wedding. Once we were all together full time, his behavior towards my son changed and over the years, grew more hostile and negative. He and my son never developed a close father-son relationship, and my son suffered because of that. Their poor relationship was a bone of contention throughout our marriage until we finally split up after 12 years.
From what you wrote about the relationship between your bf and son, things don't sound promising. I think you should prepare as best as you can to be a single parent to this baby as well. It's so new and you're hormonal and you're both going through a lot so it's hard to say "oh yeah, definitely do this" because maybe things would be better if you weren't in this situation, but step back. Give yourselves time and space to breathe, to care for your respective children, and for you to take care of yourself and your growing baby. Get to know each other better, co-parent your baby when it arrives, and give yourselves time to figure out next steps so that everyone is happy and healthy together or not.
Finally...remember that the kids come first. You have a high-needs child. When in doubt, focus on what's best for him. With that as your beacon, you'll stay on the right track.