You say "the litter box". Do you have one litter box for two cats? Each cat should have a minimum of one litter box each that is scooped twice a day, and two is better. Mine have three each and they are scooped at least twice a day. If he is having to share a litter box with Miss Mindy, he may well not like it The fact that they are friends will not alter that fact. He could be not urinating as often as he needs to because he doesn't want to use the box. That can lead to UTI's. The Cat Attract didn't work for me, in fact the cats avoided all the boxes I sprinkled it over (I was testing it for my vet) and all crowded into the one without it.

Have you tried medication? One of my cats is on Clovicalm, but there are a lot of other options. Some cats are put on Prozac or its derivatives, and some are given Valium. The medications do take some weeks to start working. Can you isolate him in a bedroom well covered in plastic until the medication kicks in?

Please try to find a no-kill shelter for Minion.