A guy his age shouldn't have any physical causes to have erection problems. The best thing to do , like they all said, is to try to work at just relaxing as much as possible and maybe cut back on masturbating.
I've had this same exact problem, just like you, and it just resolved itself in a few months with no real reason why or anything. Mine would get hard when i first started getting aroused, then go down, then come back halfway hard, then down again, and it drove me crazy. I got really frustrated.

You might also try working out and exercising more, and trying to just do relaxing things afterwards. This all sounds like easy things to do till you have to do them, but I think it's an above the neck problem rather than a below the belt problem. Maybe there's something in the back of your mind that's worrying you , and it's interfering with sex.

__________________
My threads:
http://www.pixies-place.com:81/forums/showthread.php?s=&threadid=11911
http://www.pixies-place.com:81/forums/showthread.php?s=&threadid=11261
http://www.pixies-place.com:81/forums/showthread.php?s=&threadid=8701
http://www.pixies-place.com:81/forums/showthread.php?s=&threadid=7701
http://www.pixies-place.com:81/forums/showthread.php?s=&threadid=6321
http://www.pixies-place.com:81/forums/showthread.php?s=&threadid=6378