I am sorry your dog is having trouble.
Is he urinating normally? Is there visible blood in the urine?
Is he neutered?
Is his penis sticking out of the prepuce (sheath)?
Is the blood bright red?
Are you seeing blood anywhere else (in the stool, from the nose?)?
Any bruises inside his ears, on his belly or on his gums?
Let me know and i will try to help....
I wouldn't catheterize anyway. Nearly 98% of my urine samples are taken by direct cystocentesis. This is a little needle poke directly into the bladder. Even better, is when the vet has an ultrasound to do it.
Catheterization is not usually performed as an "initial" step.