The cage is FAR too small for a guinea pig. Pet shops will say anything to get a sale. Honestly the cage isn't fit for him at all. I'm sorry. Pet shops are amazing at giving bad information just to get money out of a customer.
As said above, I'd get CC for him personally. With C and C you can make the cage any size, height or length you want. It's perfect for a small house as you can build it upwards if you can't do it outwards.
Vegetables, personally I'd suggest trying green beans, peppers (red, yellow and green), coriander - mine go nuts for this stuff-, celery and maybe some dill.
If you can't get him a friend, I'd suggest getting him a small babies teddy - no eyes or beads inside it- and he can cuddle up to that. I've done that when a piggie has passed away and the friend is left lonely. One of my current boars, even with his girlfriend loves cuddling up with his teddy bear.
You sound like a very caring piggie mother. I'm sure he's very happy with you but he definitely needs a bigger cage.