Maybe since your son is living with you full time, your fiancee could adopt him when you finally get married. That way she would then legally become responsible & it would solve that conflict. Just a thought that might solve the problem. For some women they struggle with the step-mom situation because it is hard for them to feel responsible when they legally arent.

I admire my daughter because she stepped up & was such a good mom figure to her BF'S 2 kids, but know its difficult for some to do that.