If he's separated in his own tank, he needs to have the proper equipment in it, just like the other turtle. You might bump up the water to 82F for now. Basking area temp should be 90 - 91F.
Are his eyes swollen? If so, he can't see to eat. If he can see, you could try small, cut up pieces of earthworm, which are better than the meal worms. If his eyes area closed, try holding bits of tuna or sardines in front of his nose if the water and see if he's got any interest in that.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-