I think guinea pigs are great companion animals for young people because they are usually very laid back (at least in my experience) although there is still the chance of a problem between them and the cats.

Please do not get him a bird. So many budgies have gone to the bridge because they are thought of as "starter pets" but they are not. As mentioned before they are a bit noisy, definately messy and unless you get more than one, they will be disfunctional if he decided he doesn't want to interact much with it. Budgies are highly intellegent and get a bad rap. And I won't even mention about them looking like a lovely light snack to the cats.

Perhaps waiting a while to see how he interacts and cares for the animals already in the house will lead to an easier decision process.