It isn't you, it's him and what he has going on in his head.
That being said, try this:
put a collar around his neck and inform him that he is not to orgasm until you give him permission to do so. Then do absolutely everything to him - have 30 40 orgasms yourself, use him ruthlessly - just go wild, tease and torment him, body worship, fellatio - the works; but he has to beg you to let him orgasm. Say no. Do more with him and still say no. Tease him, tease, him, tease him.
Do not let him orgasm until he loses his temper due to being sexually frustrated. Then say yes. Some times a man needs a bit of help getting over his diffidence.

> he has no problems with erection but.. he just can't have an orgasm... we've tried every position created.. even a blow job doesn't work... but he has no problems having an orgasm when he masturbates.

EEK presents some good points to consider. Here are a couple of things to answer. First, is he using condoms? If yes, then know that many a guy has trouble achieving the quantity and quality of friction required to build and then trigger an orgasm. Second, If the two of you are engaging in intercourse before he has been aroused to the point of an orgasm then you are starting too soon. It seems to be a common misconception among some young guys that the way to build arousal to the brink of an orgasm is from lots and lots and lots and lots of stroking and thrusting. WRONG. Stroking should be used to maintain a preexisting high level of arousal and thrusting to peak it bringing on a climax. If this is what the two of you are doing, then delay intercourse and devote a lot more time to making out. You want him to be on the raw edge of a climax yet not so close that he cannot still maintain control so that moving into position will not trigger it.

As fantastic as blowjobs are, they usually present a different path to Nirvana than what he is used to in order to reach an orgasm. It is not uncommon for a guy to become much more aroused from this form of stimulation than what it takes to reach an orgasm. The reason he won't climax is because the stimulation is not what he needs to trigger a climax. Soon after every guy begins to masturbate he quickly settles into a routine or a set pattern of rhythms, movements, and pressures that he then pretty much relies upon day in and day out for the rest of his life. Oh, sure, we may vary the routine from time to time in order to experiment and to stave off bordom, although if we stray too far from what works, an orgasm will either not happen or it will be less than expected.

My recommendation with regard to oral stimulation is to do it as you and he desire to drive him wild and then to add a hand job near the end in order to trigger the orgasm. This is what I refer to as the dynamic duo. Learn to mimic his stroking technique. Encourage him to demonstrate how he does it and then to take your hand and to guide your movements over a few sessions. (The same holds true for you with him.)

Back to condoms: If he is using them, then do not put it on before he is near the point of an orgasm. Another thing to do is to lubricate just the Glans of his penis before rolling it on so that his penis has some "wiggle room" in order to help transmit the friction. If the condom is acting like too much of a barrier to friction, then apply some lubricant to the exterior and let your hands build his arousal.

To cap this off, just do not begin intercourse too soon. Second, learn to mimic his hand movements so that you can peak his arousal separately and/or in tandom with oral stimulation.

P.S.

Here are a couple more thoughts:

Learn where his hotspots are and then work on them when you either want to jumpstart his orgasm or trigger it immediately.

* The A#1 hotspot is the Frenulum. This is the "Y-shaped" ridge of skin that extends from the cleft of the Glans to the top of the shaft. Rub here when he is about to climax and he will explode in no time.

* One or more additional hotspots can be found arbitrarily spaced about the Corona Rim at the back of the Glans where the head and shaft join.

* Rotate the penis in a circular movement about the base of the shaft. This will stimulate deep-seated nerves within the abdomen and quickly trigger a climax in addition to whatever else is being done at the same time.

To recap: The best way to get him to climax from oral is to combine it with a hand job.

The best way to get him to climax during intercourse is to begin when he is on the brink of a climax and not to begin early with a low level of arousal expecting a lot of stroking to get him off.

I hope this is of help. Got questions?