Personally, i would advise him the other way. Not too many cars out there that will tear it up on the autocross for that price range. Plus, the newer miatas look more like mini vipers than they do the old gay person car they used to. They have and aweful stigma about them, they they are by far one of hte best bang for the buck cars out there at that price range too. At least new...
If he is looking for a little sports car, the s2000 is a way to go, at 10grand more for the invoice price over a miata. Miatas engine is extremely strong, and begging/built for boost too from what i hear.
Now if for some reason its just the image of the miata you dont like, go for a 350z. at 2003, the special edition miata costs basically the same as a base model 350z. the Z is pimpin too. a badass car if ever there was one, though it doesn't tear up the autoX like a miata. weighs a good 800lbs more and has a 5ft wider turning radius. On the other hand, it has almost exactly twice the horsepower. Not quite the legroom, but headroom and overall passenger space is larger than the miata, which is good since you said hes a rather large man. And nobody will tease you for a 350z unless they have no idea what it really is.
http://autos.msn.com/compare/choose....odel2=&pt=dual
Use that, it is a great utility. Go check out some other models to see how they really compare.