Obviously you are just asking for our opinions here. Mine is that you should designate a time during the night that your husband will take him home to sleep. After all, what 7 year old can learn from 11pm to 4pm straight? It’s impossible. So set a time and bring him home by then if he is learning straight, or earlier if he is goofing off.
For most 7 year olds, it’s just the coolness factor of being up late or of going to the vasikin minyan. So one alternative is to promise to wake up your son a hour BEFORE the vasikin minyan to learn and then daven.