The first thing that stands out for me here if, I am totally honest, is “my” baby and “my” boyfriend. I do not get a sense of a family unit!
I do not hear you say “our” baby or “us”, as one describe a family. I don’t know if you realise what I am saying, but basically I do not read this like it is coming from a young woman who has been feeling part of a loving, connected family unit.
You describe a very loving and caring bond and relationship between the two of them. But obviously something has happened that has brought about a change in this man. You say he wanted to officially adopt your son, so it sounds to me that something has happened for your boyfriend to change his feelings towards him.
It must be something very upsetting for this young man. However, it is the child I am most concerned about as this is very confusing for him, of course he cannot understand what is happening. This is damaging for your child.
You need to act immediately and ask your boyfriend directly what is going on and what has brought about this change. You cannot wait to deal with this as your small child will become more damaged the longer this goes on.
Talk to your boyfriend because your relationship is also being damaged, but your son is my first concern.