Use tiny bits of cucumber as a treat so you can train him to do things! Mine also get a small wheel as a "night night tidbit" and can't wait to get it!
Put him on a mixed diet of veg, pointed cabbage, kale, romaine lettuce, carrot, peppers (all colours), beans, spinach, herbs like basil and parsley for every day. You can look up more in the food section or ask questions there!
For tonight, try a quarter of a carrot/half a smallish carrot and a bit of cabbage cut in stripes, see what he eats and how it goes down and take it from there. If his poo stays firm you can try him on some scissor-cut grass tomorrow morning and afternoon and his regular bowl of veg tomorrow evening. Don't push him if he doesn't feel like eating with you watching!
Has he got a pigloo or something else (shoe box, soap box) to hide in? As he is alone, he'll feel better, if he had a "safe place", especially for the night. Wild guinea pigs hide in any empty burrows they come across. They don't build them themselves, but make use of any protection they can get and love anything they can wiggle into. Make sure Basil has plenty of hay!
Over the next days watch him and see whether he can use the water bottle. Many young piggies can't. If his pee is kind of thick and dark yellow and his fur is stiff and waxy he is dehydrated, otherwise don't worry too much. He'll get a lot of water from fresh food.
Have a good night and an exciting day tomorrow!