Well, I've had my fair share of those along 24 yrs of MA training. I also served on the Reserves and I taught hand-to-hand combat to the guys. The uncooperative partner can be very unnerving and down right irritating but there is a way around that. Usually, they are the people that are scheptics. In your case, I think since he is a new guy to your unit he might feel that he has something to prove. In my case, I was one of the youngest elements in my unit so the older folk didn't take me too seriously. At my dojo I had some scheptics that felt that what I was teaching was not real or just simply, slaps. I practice American Kenpo. To the untrained it may look like a slap art and honestly, I kinda enjoy that misconception because it allows me to demonstrate on those. That is exactly how I found the "cure" for those uncooperative partners. What I started to do, in my dojo as well as in the unit, was to run the technique slow and explain it. Let the uke do his show, no problem. Then I would cover the "what ifs" that the uke tried to show off with. Last I would demonstrate the technique about at about 1/4 of power. This would either cause the uke to be in great pain or fall unconcious for a few secs but with no lasting effects or injuries (that if you don't count a badly bruised ego). This would not only capture the attention of my class, but also makes them wonder what would happen if I went 100% power on him. The respect of audience and uke was most likely. Actually, my best students started out that way, one of them said in the middle of class that the technique I was teaching could not be effective. I picked him out of the class as my uke. I did the technique at 1/4 power. Once he woke up, he became one of the most devoted students of Kenpo I've ever seen. He never doubted me again. Be careful, many people don't enjoy being slapped around a little. Use your best judgement. You can always choose an uke that is more willing to cooperate. Is funny how when people are learning football they don't mind being hit but in the MA you slap someone and they raise hell. Oh well!
Yours in the Spirit of the Warrior-Scholar,
Carlos M. Roman, H.I. American Kenpo Karate Studios