Hi Debbie. I have reared plenty of kittens with flu, and have just recently rehomed a litter of 4 who I had from 5 weeks with really bad flu & conjunctivitis. They took 2 weeks of medication to recover. Some take less, some take more. Some kittens who suffer from flu are permanent carriers.
I would persist with the medication, keep him warm and clean, and lots and lots of TLC. Most kittens taken from their mum dislike the kitten milk initially, but do come round. It's important to remember that because he has flu, he can't breathe as well as he normally would through his nose, nor can he smell properly. Althoug hhe's still very young, it really is worth getting some mushy tinned kitten food and warming it up slightly to encourage him to eat. Some kittens need a lot of encouragement, others tuck right in. If he won't eat or drin kanything and you are not confidently getting the milk into him, contact your vet, as he could get dehydrated very quickly.
Good luck, and keep me updated!