He doesn't need fruit except for a small treat occasionally. Sugar upsets a guinea pig's intestines and can cause real problems for them. But if you give him a small slice once a week, he can have it with the skin on or off.
Here's what he needs to be healthy:
1) Long strand grass hay (timothy, orchard, blue, meadow, brome) ALL THE TIME -- as much as he will eat. That will keep food moving through his system and keep his teeth ground down, saving you hundreds of dollars in guinea pig dental bills.
2) A cup of veggies every day, spread out throughout the day if you can manage that. Good staples for him are red or green leaf lettuce and bell pepper (about 1/4 of a small pepper per day). Herbs are good -- they usually adore cilantro and dill, but basil and thyme are also ok. One baby carrot per day is the limit on carrots -- they have too much vitamin A to be given in large quantities. A slice of tomato (or a grape tomato) is good. They can also have squash, a couple of green beans, endive, parsley (if they're over six months, limit that, because it's very high in calcium).
3) 1/8 - 1/4 cup of a good quality pellet per day (1/4 if they're under six months, 1/8 if they're over). Don't buy anything with colored bits, nuts or seeds in it. Just plain alfalfa pellets (if he's less than six months) or plain timothy pellets (if he's over six months). Depending on where you are, pellets ordered from (broken link removed) are absolutely the freshest to be found. Oxbox pellets can be purchased from pet stores.
Read this thread for what your pig can be fed and how often:
https://www.guineapigcages.com/threads/22156/.