All of the above advice is spot on. Bottom line is that if he is a tw@t, then you must report it as such. On one occasion I had to write an AAR that was dripping in the subject's blood - not pleasant, but as somebody said earlier, there is too much spineless tosh being written. And if everybody were to brutally honest, them promotion boards would get a lot easier.
Stick to the truth - as you (and other supervisors - you write it but don't be afraid to ask for other opinions) see it.