Seen as he's only 9 and noise, space, expense being issues get him one of these:
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This is an older model (Yamaha DD-55c...have it myself), but anything of this type from the Big 3 (Yamaha, Korg, Roland) will be fine. They're midi-compatible (you can connect external midi-instruments for more sound pallette), touch sensitive, work great with bogstandard drumsticks as well as hands/fingers, and the newer ones will have USB too (for direct PC-connection to sequencers, sounds etc).
So also quite future-proof. The 9-year old will however be happy with the decent onboard sounds for a good while yet. They've got internal speakers with volume control, and headphones option.
Around 200 of your nation's credits for new, or much cheaper if second-hand. Ebay has a few for around 50quid (assuming they work fine).