1) Get a good mouthpiece, not plastic. It makes a huge difference. A Portnoy bp02 is a good place to start, and they turn up used on eBay for $30-50.
2) Get a good ligature (thing that holds the reed on). A Rovner is about $20 on Amazon and allows the reed to vibrate freely and evenly (cheap metal ligs can pinch) and sounds better.
3) Buy a couple Fibracell reeds. They are a synthetic material that looks like a regular reed, but don't have to be soaked before each use, and don't squeak, split, chip, warp, are easier to get a good sound out of, and last for months. They cost about $13 each, but 2 or 3 would probably last him a year if he's careful with them. 2.5 would be a good strength for a beginner. If he decides to stick with clarinet, he can move to natural reeds in a couple years: they have a slightly better tone but are fragile, require adjusting and wetting, have a shorter life and you won't get 10 good ones in a box of 10!
4) Make sure that, even if he's playing an inexpensive student horn (plastic) that its a decent brand, like Yamaha, Bundy, Vito or Buffet, (not some Chinese POS) and that all the pads seal and it is properly adjusted. A horn that has even a small leak can be a nightmare to play well, play in tune, etc. and is really frustrating for a child who hasn't developed skills for delayed gratification.
5) Buy or download a clarinet fingering chart, to figure out what all those buttons do! Clarinets are complicated and there is often more than 1 way to play the same note.
6) It would be a good idea to get a beginner lesson or 2 for both of you from a local teacher, just to get you off on the right foot. Once someone shows you the right way, you'll get it. Starting with a poor setup or bad mouth position is only going to result in frustration, limited technique, and probably retraining down the road... Hope that helps! Holiday hugs, Jeremy