Why Is My 7 Month Old Boxer Bleeding A Bit After He Pees?

He probably has a urine or bladder infection it could also be a number of other things you need to take him/her to a vet or at least call one.

That's a sign of infection or crystals or stones somewhere in the urinary tract. It's usually becoming pretty severe and possibly spreading to kidney's or bladder if there's blood evident. It requires antibiotics to treat and only gets worse if left alone (and can even kill them).  Your vet can do some quick testing to figure out which type of antibiotics are going to be the most effective for treatment. If there's blood it may take more than one course of antibiotics so be sure to go back when your done with the first round to be sure it's completely gone. Hope he's all better soon.