You can give him chew sticks right away. Whatever kind you give him make sure he's not eating large amounts of them and not eating enough of his regular food. If that looks like it's happening give them as an occasional treat. He should have chew blocks and pumice stones available at all times just like an adult. Until he is full grown, which occurs at about a year, just give him half the treats you would give an older chinchilla. You can expect he will grow quickly at this time and nutrition is very important at this stage.
Typically owners who pair chinchillas will put them in a cage next to each other (no closer than 2" together) for two weeks then after two weeks let them play together and see if they are aggressive towards each other. If they become violent (pulling out hair,threatening to bite, etc) then wait another week before trying again. I have no experience with this myself but from what I see on the board it seems to work in most cases. You'll have to be especially careful since the smaller animal may not be able to defend himself as well against the larger adults.