Mr. Brown has some good information (as usual ) on teaching kids to ride.

Does your son have much experience with pedaling in general? If not, you may want to familiarize him through something stable (more than two wheels).

If so, I, personally, would try adding something silly to your routine to not only lighten the mood (and make you feel better) but also so that your son doesn't equate cycling to frustration.

Continuing the idea of the grassy field, I may tell him that for every so long he is able to ride, I will fall over on my bike (my beater). I'd probably do so in a silly and exaggerated manner, in a way he would find hilarious. If he is getting frustrated with the falling (not necessarily related to your frustration) it may also make him feel better that he's not the only one. Hopefully it would motivate him to try to stay up. Of course, this is just a random idea. Your son may react better to a slightly different situation, but the idea is there.

Have fun, its important.