Can Erectile Dysfunction happen 'sometimes' and not all the time?!

My boyfriend and I have been dating for a year and for the past 4 months we haven't had sex.  He said that he might have ED.  For the first couple of months the sex was great and often but now it's nonexsistent.  My bf says that he can't help 'it' and doesn't want to talk about it.  When I ask him to try and explain what's going on, all he says is "It doesn't work very often!"  He is only 34 and I know he masturbates so why can't he seem to get an erection w/me?!  Is it possible that it could really be ED when in the beginning there didn't seem to be a problem?!   Help!