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Was he dewormed and examined by a veterinarian when he got his most recent vaccines? Is he gaining weight every week?
Hmm. We can't completely rule out the possibility that his personality may just be more subdued and may always be a skinnier cat.
However, deworming is really important in a young cat. They almost always have worms from their mother and those worms can make them thin and lethargic. Feline Leukemia virus and Feline Immunodeficiency virus should also be considered for adopted kittens.
A change in diet may also be in order. Some foods are just not that good and may not provide optimal nutrition for him. It's also possible that the food may be great, but not great for HIM. Meaning, for whatever reason he can't digest it as well or he may be sensitive to it.
I think your first step, if possible, would be to head to your vet to get some deworming and talk to them about Feline Leukemia and Immunodeficiency testing.
That is good news. Those are nasty viruses and it's nice to know they are safe.
I would also try some dry food as it tends to be more calorie dense and he may gain some more weight with that.