depends on how much he uses his wii obviously. every day seems like too much but you could be using some not very high quality batteries or he might play a lot. but either way, the best way to go is just with a charger. with this one you just set the wiimote in and it charges. i have it and it works great. completely cut out batteries. check it out
You could also use regular rechargeable batteries too. I already had a charger so I bought enough batteries for 4 wiimotes. I've been using them since I got my Wii at launch. I don't feel that it's too much of a hassle to take them out and put them back in. The Wii in my house is used pretty much everyday and I usually charge once a week for an hour. Up to you.
The one's suggested by Barris are a lot more convenient but when I got my Wii they weren't available and I haven't bothered to make the switch yet.
For Christmas my brother gave me the Nyko rechargeable batteries and stand that Barris linked above. I've been happy with it. It's a slight nuisance to have to remove the plastic jacket every time you want to recharge the thing, but on the other hand you'd have to do that anyway to change the batteries, so I don't consider that to be a valid complaint. I don't leave the charger unit plugged in when it's not in use because there is no good place in that room to store it that would not be in the way; instead I unplug it and stick it on top of the Wii (which I have lying flat due to space issues around the TV) and then pull it out when I need to use it.
COLDshiver's point is also very good, and if I were buying the batteries myself I would have done what he describes. My PC has a wireless mouse that runs on AA batteries and I bought two sets of rechargeable Duracells for that—when the batteries die, I pop in the other set and throw the dead ones in the recharger, then when the light turns green I unplug it and set it aside until it next comes time to change the batteries. My electric toothbrush is likewise battery-powered and I'm going to use the same method for that. I would do the same for the Wii had my brother not already given me the Nyko unit.
The rechargeable batteries are pricier at first blush than the single-use ones, but you'll quickly make up the difference. I would also suggest that you check the Sunday newspaper coupon section. Sometimes they have battery coupons in there that you can apply towards the rechargeable batteries (and if the batteries at the store have another instant-use coupon on the package, so much the better).
The current rechargeable batteries are FAR better than the Ni-Cad batteries that were available 20 years ago. If you're concerned about the memory effect that used to plague Ni-Cads, you need not be.