From a Desperate Gf, 21yr Old Bf, Viagra, Not Working!? Help

Short sweet and to the point. My bf is 21, his had been given a sample of viagra bc he cant keep hispenis hard when we are about to have sex. By the ways hes a virgin. Every time we are about to do the deed (with out theviagra) is goes down. He thinks its a mental thing bc he nervous and his mind is so many other places. And we have been goingthrough this for 2 months ways to get around it, I mean every ways you can think of to calmhim down, take his mind off the pressure ect.. So, then he went to the doc. She gave him viagra. He took it once, had all the side affects ect... But once again, his penis was har, but the second the pants come down or we more to have have sex, right back down!! But on viagra?!?!Whats going on in his head? Is it mental or physical at this point? Or is he gay? (doubful but im runnin outta answers) and the sad thing is he does it fine by him self, in fact that night after I left he masturbated!! Anyone have any answers!?!? Please help! Thanks! Ps I have been sexually active and and love my bf very much and want to take it further and this is really stressin me out!! Mentally, physically, and sexually!! I might loose it!! =)