If he is older as you say, he should be eating a lot more than 1 mouse every week and a half. It also sounds like your temperatures are off. I'd suggest reading the care sheet on this site and following it so the husbandry and feeding are correct, then see if his condition improves. You should be able to tell if the black specks are mites. If they are you should order some Prevent-A-Mite and follow the directions on the can. If he isn't eating or seems lathargic, take him to a vet to see what's going on.