There are lots of things that can cause bloody stool and a fever. The three that come to mind for his age are:
Parvo
HGE (hemorrhagic gastroenteritis)
Pancreatitis
If your vet has not done bloodwork yet, this is the next step and should tell us what is causing problems with your baby. If he has not already run a pancreatic specific lipase test...have him run a lipase test when he does the bloodwork.
Parvo will harm pets that have not been fully vaccinated (at least 3 DHLPP vaccines). Mostly it's puppies. The oldest dog I've ever seen with parvo was 6 years old. There's a test for this affliction. Bloodwork might also indicate infection.
HGE and pancreatitis can be caused by feeding your pet human food. Both pretty much show up the same but pancreatitis can usually be found in pets with greasy stool (both contain blood in my experience).
I would hope by this time that your veterinarian has placed your pet on IV fluids to rehydrate him as well as medications to lessen his pain if he appears uncomfortable. In most cases with rehydration and time, most pets will come out of whatever is ailing them...but it also helps to know exactly what it is that is going on.
Good luck!