New to guinea pigs. Tips appreciated

Hi! So i'm considering to take care of my cousin's guinea pig. My cousin absolutely doesn't care about him at all, and even though i prob cant provide what's best for him, he's prob in better hands compared to when he's back under my cousin's care.

Currently:

• 70x40cm cage (its pathetic, but his original cage is half the size. This used to be my old hamster's cage so i used it for him)
• bedding: pee pads. I change them daily.
• water: water bowl. Used to drink from his water vottle but since coming he refused to drink from the bottle and prefered the bottle (my old hamster's food bowl)
• food: still feeding him timothy hay, pellets and sometimes vegetables. He has never had veggies in his life so i'm ensuring to not feed him too much in case his stomach can't adjudt to too much fresh food. Only issue is that the pellets have seeds in them which are bad, but for now i try to remove the seeds from his portion. Currently not giving him any vitamin C since i was only suppose to take care of him for 10 days while my cousin's away...
• hideout: i have one clear acrylic one for him, it used to be my hamster's platform but it's the only thing i could find that fits him.

Obviously not the best cuz i feel so so bad for not being able to provide him with a partner, a good sized cage and all. But i still think its better to take care of him since he's neglected by my cousin.

Any other tips i missed out on? Would really appreciate, esp with what i could do with the small cage size. I currently have a rabbit thats freeroaming, so having a bigger cage isnt rlly an option since it prob cant fit in my room (my rabbit can't go there) Also do they need enrichment? And if so, what kind?