My cat is having digestion issues?

Just some background information: He's a 12 year old American Shorthair and a bit over weight but not terribly fat, he is both an outdoor and indoor cat, and he has no record of illness or injury.

Recently I've been finding blood in his poop, so I brought him to the vet and they said to feed him wet food instead of dry, which worked for awhile but hasn't been very effective lately. The blood problem got really bad so I bought some kind of stomach sensitive food for him which seems to work but he really detests the taste of this brand and he only eats enough to get by. Because of this he's lost a lot of weight in the last few weeks, so I switched back to his normal food. He's eating more now but now I have the bloody poop problem again. Also he's been meowing more frequently than normal and it almost sounds as if he's trying to tell me something is wrong.

I realize he's getting old but he does not appear to be dying at all, he still has a lot of energy and doesn't look like he is sick. The vet has no clue what is wrong so hopefully someone can tell me what to do