The nasality is a resonance, but it's a harmonic one, with multiple peaks. One trick (shh, it's a secret) is to use a comb filter (flanger with sweep width and feedback set to zero). Play with the delay and wet-dry mix until it's tuned in and the nasality just sort of "smooths out". The delay should be short, around 2 msec down to 0.5 msec, and the mix goes from 100% dry (no notch depth) to 50/50 (full notch depth). This, with some EQ for overall shaping, could do the trick. But be careful... if you remove too much nasality, he'll sound stuffed up, like he has a cold.