Why does my Doberman have big red blood spots or bruising on his belly and sides?

You would want to see your vet for that to make sure something major might not be going on. You wouldn't want to blow off any sort of internal injury or damage and if your pups parents/line isn't VWD (and other hereditary problems) clear it is something you may want to look into, possibly check things like thyroid function as well. You would want to catch a bleeding disorder or other underlying problem asap so you are aware of it. Your vet should have a better idea if some underlying disorder should be looked into after they are able to check your pup out and speak to you about everything. The first link is the breed parent club for you. Hope your pup is okay and it's nothing too serious.
www.dpca.org
www.wildheirlabradorrescue.org 
members.tripod.com
www.petfinder.com