K.F.
I understand your pain. When I was a single parent as a rule, I never let a love interest into the life of my child unless he was more than just a boyfriend. You don't indicate how old you are so it is hard to guage the time factor.
I'm married now and 44 years old. So at some point in my adulthood I decided it was more important for me to like the guy, develop a relationship with the guy, learn his character and habits which takes over a year, and then possibly introduce him to my son who is much older than yours.
I hope you aren't living with your boyfriend. My great-grandmother always said that playing house is dangerous for us women because we can distinguish fantasy from reality in our hearts. I didn't understand her while she was telling me this at 12 but I came to learn the truth of this statement.
I would encourage you to try to do as much as you can for yourself and your son without BF. Act more like a single (I'm not talking about dating others). I'm talking about being responsible for you and your son's well being. Why should your BF be allowed to have a relationship with you and your son where he doesn't know where it is going?
Don't you dare say a word to BF. Just show him life without you and your little one and see what happens. He's been your BF for 3 years and I think that is plenty of time to know if he wants a commited relationship with you and all the responsiblity that comes with it or if something else is going on here.
Before you invest more of your or your son's heart in this, kindly withdraw, wait and see. Maybe he isn't the kind of man you want to have father your son. I hope this wasn't too harsh but helpful.