There's a scent that dogs give off after being outdoors that is a little stinky. It goes away 15 minutes or so of them being outside. Little kids have this same smell after playing hard outside.
I read about once, but can't find the link. It's a real thing and they think dogs do it to tell other dogs in the neighborhood that they are around. Anyway, for something like this there are lots of temp sprays and fresheners that are safe, but this smell goes away on its own.
If you think he has a real odor problem I think you should investigate. He should smell good a least a week after his bath. Sources of bad smells can come from filled anal glands, which can be dangerous, they could rupture. Also yeast infections can give off a funky odor around ears. Rotting teeth can give off bad smells, so it really behooves you to get to the root of problem, and not just mask it.