Unfortunately, this is happening all too frequently with child-rearing changes in values and lack of healthy parenting especially after a divorce. But it can happen with widowed people as well.
I strongly recommend you trust your feelings to back off for now and encourage your boyfriend to get solid counseling both for his role as a parent and the ongoing relationship with the son. Better yet, the son should also get counseling and a meaningful task--if not a job, then an educational program.
It may come obvious that the boyfriend is more into his relationship with the son than into you. In that case, it's better to know that sooner than later.
Best wishes, to you, Dorothea. www.secondchancedream.com