I have had a couple of long term relationships that were like this and the reasons were :
the hubby ) he had been abused as a child by a woman and was unable to ejaculate inside a woman ( but could do the above stuff ) cos it reminded him to the woman who abused him
the boyfriend ) he was battling with his homosexual feelings and his other need to be in a couple relationship
the friend ) his adoptive mother used to punish him by spanking him on the bare bottom with a hair brush - upto the age of 16 ! and now cant get an erection.
there is of course a common theme and might be pertinent to your guy, but may not.............hope it helps..........
good luck