Kate McCoy's advice is totally accurate, and I second the notion that couples counseling would be extremely valuable, esp because it would remove any presumed bias about who's right and who's wrong. If your BF would be more comfortable with a man (because of unexpressed assumptions of bias) seek out a man.
I just want to add an additional dimension: You might both have idealizations lingering from your HS relationship. And if it lasted 2 yrs or more, it might have set up a kind of unconscious control-mechanism (like we have in cerebellum for riding a bicycle) for how relationships are supposed to behave. Yet you have both matured quite a bit since then, and you may not be as much like the teenagers you were then as you're unconsciously assuming.
In addition to that, you have long marriages to set up probably MORE potent control-mechanisms for how relationships are supposed to behave. I actually think it would be really GOOD for you to negotiate your way to marrying each other. For you have a different model for what marriage means compared to what "Dating" means. Getting married would be the most succinct and symbolic way to declare the primacy of your couple relationship over his codependency with his son. THAT's a primary subject I would advise you to bring up in couples counseling. (Somewhere behind his codependent relationship with his son may lurk a complex problem with HIS exwife that he's not dealt with, and that's likely to stand behind his refusal to marry you or to give up on infantilizing (OK call it adolescentizing) his son.
I have to go. Ask any more questions you want. You can converse with both Kate and me, and it doesn't matter which of us you release your deposit to, since it's your satisfaction that the company seeks to maximize.