Advice please!

Dixie is about 6 weeks old, a rescued feral kitten.  We have been handrearing him from about 2 weeks old. He has always been a greedy kitten, attacking his food as if it was his last, firstly his milk, and more recently his solids. Last week he went off his food and refused to eat or drink for about 12 hours.  I took him to the vet, who could find nothing wrong with him (except that he does have a very large hernia, stretching from his left side, just above the hind leg, to his groin, next to the hind leg and will require an operation when he is big enough) The vet gave him some injection, dewormed him and gave me some antibiotics, just in case he missed something.  When I got home, Dixie attacked his food and ate 2 bowlfuls.  He was fine for a week, and yesterday he refused to eat or drink again.  I am at my wits end because in a day he went from a fat, round kitten, to a thin, scrawny thing.  I cooked up a chicken neck and gave him bits of meat, which he ate.  I also used a syringe and gave him some milk last night, which he also took, without too much fuss.  I have been feeding him Whiskas kitten   pouches. Is he just being fussy?  Should I rather give him cooked chicken rather than the Wiskas?  Maybe a multivitamin to try and stimulate his appetite?  All advice will be greatly appreciated as I am very new to kittens.

It could be caused by the hernia, but the vet says there is nothing they can do until he is about 3 months old.  I noticed that he did eat a bit of the Whiskas this morning, but not the amount he usually eats.  I have been giving him a kitten formula, not cows milk. I will go and speak to the vet just now, but I have not much faith in them, they are not very good with small animals and unfortunately they are the only vets in the area. I am looking for another vet in a bigger town to do the hernia op when the time comes.