Ah, you have located one of the blunders of WoTC regarding the monk class and their complete lack of feasible flexibility within the class, not just in multiclassing.
I guess the big change that 3.5 did that gives monks an advantage is that they are allowed to, within their attacks,
interchange weapon strikes and unarmed strikes FREELY. This is something that many players probably do not take advantage of.
So, say your monk, with flurry of blows, had 3 attacks at +9/9/6
Your first attack could be with the kama at 9, your next attack a kick at 9, and then your next attack another kama strike again, or another unarmed strike.
A strategy you could use with that is to give enchanted monk items that give you defensive bonuses of some sort. Like one of those "Defending" weapons where you take the enhancement bonus and add it to your AC instead of your AB in given arounds. So long as you're wielding the weapon, he can still use the item, and then throw your viscious kicks in at the same time.
Granted... all of this isn't a really big deal. A kama gives you a +2 to Trip, so you trip on +9, get the free attack (smash em with a kick), then do something nasty like Stunning Fist them the next round (or you probably can't, so just kick em again) and then... uh yeah, kick em again.
The only monk weapon that is entirely useless is the quarterstaff, and perhaps the siangham, since both weapons do not provide a bonus in any sort of maneuver the monk does.