Keep in mind that fresh red bright blood indicates it is not from the digestive system but from the mucous membranes in the intestine. This is much better. It could be that he has a crystal or something which is causing a lot of irritation in his bladder and urinary tract which is just upsetting his whole digestive system and causing the small intestine to spasm more which then is irritated and can bleed easily. Or he just had a hard, large bowel movement which tore the membrane a bit and it bled. I don't think you said whether the stool was hard or soft.

It is when the blood is dark and makes the stool very dark and "tarry" and stanky that you panic more. Then it is from the GI system.

Prayers for your sweet boy and I hope he is feeling better soon.