Wethers don't have teats
They are castrated male lambs, only does or doelings have teats...
Are you sure it is fleas? With all my experience with goats. sheep, cattle, horses, pigs, etc. I would say he just has dry skin or a parasite.
To treat dry skin, give him little capsules of fish oil in his feed. If it is a parasite, try deworming him and taking him to the vet again to get checked out for internal parasites...
If fleas are the problem, you can check by separating his fur and looking for tiny black bugs crawling and JUMPING. Fleas jump alot. Mainly look on his belly and at the start of his neck. I suggest also checking for ticks on his neck and back.