I have a two year old tabby, when we first got him, he had constant diarrhea and would not gain weight. I posted this question and was told to put him on a grain free food, and it worked great and quickly, that was probably a year ago. But, for the past two days he has had diarrhea again. The first day was bad with blood in his stool, and some vomiting. Today was just the diarrhea, I have not changed food or given him anything other than his food, he is an inside cat, and is still playful.
Does anyone have any ideal what this could be?

Try putting him back on the grain free food? Or just take him to the vet - he might have some issues that need to be fixed immediately!

My guess is worms..

When was the last time the cat was at the vet for a fecal or gen. health check?

That's where I would start. Food wouldn't create issues with throwing up and bloody poo, but worms or other types of parasite would.