Hi Lydia, welcome to the forum, You will find alot of information on here about different aspects of guinea pig care, If you are feeding him dry food, make sure it is guinea pig mix or pellets, as they contain added vitamin C which guinea pigs can't produce themselves. Lots of hay and fresh water should be on offer to him all of the time, hay is used to keep there teeth in good condition and is also good for their digestive systems. As for fresh veg my guineas get quite a selection in a salad dish every day, carrot, cucumber, green beans, celery, tomatoes, kale, sweetcorn both varieties, peppers, grapes to name but a few. You say that he seems to scratch himself, does he do this alot? Does he have any dandruff at all?
Do you have just the one guinea pig or does he have a friend, guineas are social creatures and love to have company of their own kind. People are not always made aware of this when they go to purchase a guinea pig. What is your guinea pigs name?