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What does he eat as his regular diet and treats?
Is he short haired?
How is his appetite?
Are bowel movements normal?
The most common thing to cause vomiting in a young cat like this is a hairball. He may not be spitting up any hair but that can still be the cause. It sounds like you have him on a good diet and everything else sounds normal, so this is most likely the case. Some cats groom quite a bit and ingest the hair - this builds up in the stomach and causes vomiting. Sometimes they get the hair up, but often it is just vomiting. What you need to help with this is to give some laxatone, or hairball remedy, to get the hair worked through the GI tract and out in the stool. This will clear the stomach and stop the vomiting reflex.
You should dose him with the laxatone 1 teaspoon twice a day for 5 days and then use the maintenance dose listed on the bottle. You can buy this over the counter at any pet store or vet's clinic.
Let me know if you have more questions,