Hey Debbsy glad you've got this resolved for now. I can identify with your problem from a different angle, that of the child. My father has an all-consuming hobby, steam railways. When we came along, I know it caused huge resentment that he disappeared for many weekends to go and play trains as my mother (and later on my stepmother) saw it. Particularly as when we were small, he worked away from home all week.
It was very hard for us children, all the excitement of 'daddy's home' followed by the let down of him packing his bags and buggering off again. And I can completely sympathise with my mother and stepmother. But this is something that is in his blood - he's been involved since he was a teenager. I don't think he could give it up.
On the plus side, we sometimes got to go along too, which was fab (and made me the envy of all the boys at school, especially firing the Flying Scotsman and Mallard). We got a lot out of it too. And I have really enjoyed rediscovering it now I have my own ds and can take him along occasionally.
I think my dad was selfish when we were tiny not to cut back on it. He went away too much. (He is quite a selfish person, tbh.) But I don't think it would have been reasonable, or possible, for him to give up completely. And I'm proud of him - he's one of the leading figures in his field, a recognised expert.
When fatherhood becomes a reality for your dh he may well want to cut back of his own accord. It would be wrong if he didn't, to some extent. But I don't think you should ask him to drop it completely.
What winds me up these days is knowing he's left all the very expensive engines he part-owns to a steam preservation trust - there goes my inheritance!
HTH