As your son is separated from his wife, there is probably no liklihood of him reaching an agreement with her about this, as she already has refused to install parental controls on the children's phones.
Please, do not get directly involved here, apart from advising you son to state quite clearly to his children that when they are in his house, they obey his rules.
There will be tantrums, when he makes it clear either that their phones are to be locked away by him, when they arrive and handed back when they return to their mother's, or locked away at a certain time every evening.
He will probably have a difficult conversation or two with their mother too,
Right now, he is making a door-mat of himself, and as many divorced, or soon to be divorced parents do trying to "make up" for the split with their mother by spoiling his children rotten. He needs to put his foot down now, install netNanny on his own laptop, and explain to his children that they need a decent night's sleep, which they are not getting if they are playing games till all hours.
Disconnecting the internet at a certain hour sounds a really good idea to me.