With all the things she does for him, in all the ways she is there for him, she is a parent to him. She will never be his mom. This much is clear. But there are probably so many similarities in the things you both do for your son. If she is not a parent to him, what is she? His maid? His cook? His chaufer? His doctor? Do you consider yourself those things (ok, we probably all do at some point!) but you know what I mean.
I guess what I'm saying is that it seems you are discrediting her role. You describe her as a good stepmom, she knows her role, respects the boundaries, loves your son, respects you... why not give her the credit she's due and call it like it is? It takes nothing away from you to admit she is a parent to your son. How lucky a kid is he? 3 parents that love him and only want what's best for him?