Scolding, yelling, crating, none of that will work with a cat.

Here are some things to consider:
1. Changed the brand of litter? Cats can be very particular about which litter they use, and a change can trigger this sort of behavior. I've had 2 cats who would only use Tidy Cats scoopable, no matter the cost that week to my wallet, lol.
2. Take the lit off the one litter box, see if he starts using it with no cover on
3. Clearly location of the boxes changed when you separated. Are the boxes in the place they were prior? Cats sometimes insist you not move their bathroom.
4. Is a food and / or water bowl anyplace near the litter boxes? Cats do not like their dining room near their bathroom. These need to be at least 8 feet apart, for some cats.
5. Are all the cats neutered and spayed? Especially the youngest, age now 4 years? If not, get that done.
6. are all the boxes lined up in a row? Could HE be trying to bully the others, claiming all the boxes as HIS TERRITORY? Or perhaps one of the others does this. Can you move some boxes further apart (if they are all lined up) -- and still have all of the boxes away from the food and water bowls.
7. If all the above have been addressed, buy some Feliway, and use that in 'their' room. See if that makes any improvement.

Good luck!