It may be that he is just too busy with other things to think about eating, some puppies are like that and if you pamper to them they will grow up into fussy adults. You cant change a puppys food overnight as it must be done gradually, but if you think he doesnt like the food you could try adding something else - tripe always goes down well with puppies and none of mine were able to resist it, but I only give them small quantities in with their other food.
What are you feeding him - he should still be on 3 or 4 meals a day, and personally I feed two meat meals (complete food with some fresh meat and veg) and two milky meals, either rice pudding or cereal with goats milk.
Is he drinking ok, and is he pooing normally. If you are worried it wouldnt hurt to give the vet a visit, as he really needs his food for his bones and body to grow otherwise he will become quite gangly.