Hi Taggie,
I'm so sorry your boy is not well.
I had to post as it sounds like you are having the exact problems I have had with my boy. He's always had sensitive guts but the anti-inflammatory they prescribed him (Trecoyxcil?) caused him to progressively bleed (to the point of needing a transfusion early in the new year).
I posted a thread on here which you can find with a search.
My boy is also 13 and I thought it was game over. This had been going on progressively since November (the anti-inflammatory was being given once a month and was killing his insides).
Do you know how long he's been bleeding for? Are his gums and tongue paler? Energy okay? Apart from the bloody poo, I noticed his energy was gradually flagging (due to losing the blood).
If the bleeding is recent, I honestly think there's every chance you can turn it around, but I suspect you need to get the anti-inflammatory out of his system. (Ours took around a month).
We still have bad days but with antacid (omeprazole), stomach protector (antepsin) and very careful diet he is now acting like a 2 year old, unbelievable!
Another recommendation from Behindlock was to buy Tree Barks Powder from Dorwest Herbs - it's really helped settle his stomach.
Sorry if this is overload but I just wanted to let you know that it's not helpless at this point, and to share my dog's experience with you as it may help.
Best of luck and I hope he improves soon
xx