I don't know how much it is due to genetics, metabolism, hormones or whether it about the relationship we have with food or a combination of everything. It is interesting to see what and how slim people eat. My husband is slim. He eats little and often. He takes forever to eat, which for me gets very boring! He is the slowest eater in the world! I leave him to it if I can. Apparently very healthy to be like that! He eats very little rubbish. I like making cakes, but I can't eat them and he's not that interested. He's lucky.
I think the way we think about food is passed on to us by the way we are brought up and passed on through the generations. Being born not long after the war, while rationing was still going on and to a mum who had experienced poverty, I was told I was naughty if I didn't eat everything up, even though my body was telling me I had eaten enough. We had a lot of battles over food. She has since apologised to me. I have a really lovely friend who is morbidly obese. She says her mum her mum associated love with feeding her.
Your husband has done so well, but as you say, maintaining his ideal weight is an ongoing battle. What kind of support does he have? Obviously, he has yours. Just a thought, though not a personal recommendation, more here-say, is Paul McKenna's book on slimming.