How to get a kitten to eat and when to be worried

Yesterday I brought home small, 3 months old stray kitten and I can't get him to eat.

We were at the vet yesterday, he got a parasite medicine around 7 p.m. He ate RC kitten wet food around 8 p.m, half of the bag I'd say and then threw up around midnight and three times during the night (wet chunks of food).

Today he is eating very little dry food (maybe 5-7 grams), he won't touch the wet food. I even bought a kitten milk formula thinking he might want that better. He's purring, playful, wants a lot of cuddles and generally seems good. He did drink some water after he figured out how. He poops normally, no diarrhoea.

It seems to me the problem is he doesn't really know how to drink or eat or be a cat really. He was found in the middle of nowhere, alone, with no other kittens or mother in sight. I think somebody dumped him there. Any advice? When to seek vet attention again since they told me to come in two weeks yesterday?