I have two step sons. Both from different Moms. I have one Son with my husband. My oldest ss was 9 when i met him, my husband has been in and out of his life (his Mom and my husband had a rocky relationship which lead to her not letting him see my husband during a majoity of his childhood). When we met, my husand had always had my younger ss, every weekend, holidays, and summers. My husband and his Mom got along well and communicated well. Night and day literally. When i came along, my older ss was already grown up. I mean at 9 he is already set in his ways. My little ss was 3 so he was still small.
We built a life together, included both boys in absoultely everything we do. My older ss was always with Mom, always prefered Mom, and just came to visit Dads, and come for the fun stuff. I can try with all my might, we can do everything in the world and he will always feel like our house isnt his house. He doesnt stay for more then 1 night at a time, then cries for Mom (he is now 13)
My little ss is the complete opposite. He always feels at home at our house, he says he has two homes. Two everything, he lives with us every weekend, every other holiday and every break from school & summers. So i get where he is more comfortable. We have done everything the same with the two except their Moms relationships are different with my husband and I. So i don't know what it is but i understand you.
My parents are together 33 years, but even at that I still hard time calling my MIL "mom" or FIL "dad" because to me they aren't Mom or Dad so its hard to force it. Neither of my ss's call me Mom because i'm just me. Not Aunt just me and i'm ok with that. As long as they treat my Son like of of them which they always do, i'm ok with that, my home is always open to them and i love them unconditionally as i'm sure you love your ss.