Can you take pics of the spots? Are they raised spots like hives or just spots of missing hair? Hives can be dangerous, but since he's been to the vet he should be doing okay... bald spots could be many different things. It could be fleas (did you seen any fleas on him?) or it could mean a skin infection such as demondex mange. Mange is manageable if you catch it early enough... did they do a skin scrape on him?