Has the dog been thoroughly checked to make sure there are no ticks still on him?? (and I mean every part of his body, the little buggers can get everywhere!)
where do you live? do you have brown ticks in your area? the reason I ask is because off ehrlichiosis; when it comes to ticks most doctors will check for Lyme disease but not for ehrlichiosis. In fact some dogs could carry it for life and not have any symptoms.
Good luck, and I hope the little one gets better soon