I suggest you slow way down on this. You and your boyfriend have only been together a few months. It’s very soon to be making a life-long commitment, much less to involve the little boy.
I’d go a step further than your therapist. The boy’s parents split less than a year ago. That’s almost a third of his little life. Many kids are still adjusting to the change in the family at that stage.
Without talking with him and his parents, I don’t know if he is feeling at all abandoned by his father or worried about his mother. Another consideration is that little children open up their hearts to people quite readily. It’s unfair to involve them in creating a new family until it is 100% certain that the relationship between the grown ups is stable. If you hurry into his life and then something were to happen to cause you to break up with his father, the boy would again have to deal with the disruption and feelings of abandonment. That’s not fair.
I understand that you are eager to get your life with your boyfriend started. I suggest you wait until there are concrete plans for a wedding or legal commitment before involving the child. Going slow now will pay off big later.
As for steps to take for introducing yourself into his life: Do talk this over with your therapist. She knows you and is in a much better position to advise you.
I wish you well.