This may just be a normal discharge called smegma. If it should change color or have an odor, I would be more concerned. As your dog is being treated by a Vet already, wouldn't hurt to mention it to him.
Is he licking himself frequently?
Infections in dogs' penises often cause such a discharge - possible, I guess, that he caught it while at the vet's if he wasn't having the discharge previously (you vet should check that bloodwork to be sure it wasn't already showing up. Probiotics shouldn't cause it; they'd affect his gut flora, not his urine.
You should call your vet - it will probably need to be checked to be sure not a bacterial infection. If it is an infection and not something exotic, usually Clavimox will cure it - or has for our dogs, at least. Clavimox is not an expensive drug, and is a pretty broad-based general antibiotic that catches most dog and cat infections.