yes that is very common. My second language is English and sometimes I stutter more in english than in my native language. Maybe in your case it's because of lack of proper practice of the language. The next time he stutters try giving him a gentle reminder to talk slowly if he doesn't and to trying prolonging the words as much as he can. Try finding a way to cure his stuttering at a very early age as it might get worsen for teenagers as they grow up because of stress, anxiety and peer pressure.