When they start passing remarks, respond with something bland, like "Thanks for voicing your concerns" and then change to the subject to talk about the weather.

Both my DH and I think that we succeeded despite our parents. We both have our issues that we are aware of and are trying not to perpetuate them. 

In-laws are always going to have something to say especially in a step situation where they feel extra protective of the stepchild even if you have adopted them. Just have confidence in your own parenting ability and have the confidence to ignore their comments or keep them at a distance if they are trying to meddle in your home life.

Your DH needs to tell them to butt out, you two are capable of raising him just fine without their input. 

So 1) I remember you! <3 

2) I'm with everyone else.  DH needs to tell them to butt out and then it's grey rocking.  No reactions and just keep doing the awesome job you do with your son

No, it doesn't end, at least not without some hurt feelings and possibly resentment.

As with your DH, my DH's family was heavily involved with firstborn OSD. He was a dad and married at only eighteen. The newlyweds lived with FIL and DH's sisters, who all doted on OSD. Between work and school, DH allowed himself to be marginalized. He became the outsider, while THEY were the core family, which led to many problems down the road. 

It's as if these family members cling to a dynamic that no longer exists, and while it affects YOU, only your DH has the power (and responsibility) to change this. He needs to have firm boundaries, challenge their thinking about what's best for his son, and insist they support rather than subvert. You should keep things light, superficial, and distant with them. And sadly, be prepared for your son to possibly choose them when he's older. I wouldn't be surprised if low level alienation has been going on all along, with them thinking of ways to "rescue"  your poor son.