It depends on what your opinion is about the movie and how mature he is for his age.If you think it is too mature for him then I don't think so,but if you think he can watch it and still be a sane person I'm sure it would be fine.
Do a google search of 'why is step brothers rated r' or whatnot. There are sites that will tell u the exact content causing its rating without spoilers or having to watch it first. There is some 'saucy' language and some pretty obvious sex scenes and general bad behavior. I found it hilarious but I'm also 27 and not going to try to repeat any of it haha