" I'm dreading (?) the day he comes back saying his dad says he'll die at armageddon if he doesn't become a witness".
Simply reflect the issue into a level that is appropriate to his understanding.
Sort of like "What a very silly idea! God is not a monster. He loves the little children".
When he starts school and as he grows over the years make sure he can enjoy the social aspects.
Encourage him to enjoy the present.......... but also to look forward.
If his JW father becomes hurt full or insistent that he becomes a JW I have heard about negative JW issues looked at by family court judges who restrict the JW parent and tighten up on visits.