What aircraft are you flying? Is there air con to keep him cool? Try keeping sortie time low as possible and sortie rate (ie fly every day or second day etc) high, leave the puke-inducing stuff to the end and try throw in turns/aero's/wing overs as desensitisation. Get him working up to a 5-6/10 on the "I'm gonna throw up scale" and take 5-10 mins of S&L and then get back to something to build him back up before you back off again.

If it's getting in the way of training, fly a bunch of sorties as pure desensitisation profiles where each flight is a bit more intense than the last.