I don't know how the gym's are outside holland but if he can get a decent small class to train MMA then I would say that's better than a traditional martial art. Only Muay Thai would be as good as MMA imho, because it's probably more effective in a real live fight.
Besides that boxing is the most fun. Judo and Jiu Jitsu will make you strong, but they're hard to learn when you're older. So as long as he starts with striking I'd say that's the best. If he doesn't want to do striking, he can do judo or jiu jitsu. I just want to spread info.
Of course it's important for him to keep his body type in mind. If he has a low center of gravity judo/jiu jitsu will be very effective.
Besides that boxing is the most fun. Judo and Jiu Jitsu will make you strong, but they're hard to learn when you're older. So as long as he starts with striking I'd say that's the best. If he doesn't want to do striking, he can do judo or jiu jitsu. I just want to spread info.
Of course it's important for him to keep his body type in mind. If he has a low center of gravity judo/jiu jitsu will be very effective.