polarbear has a lot of experience with this, so I would give serious consideration to the above advice.
I will add one thought, though, on transfer and generalization. If you see that he can do the printing in isolation, but that it doesn't transfer to live writing, you might consider if enough deliberate transfer training is occurring. I don't know how your OT is having him practice, but it is important to move from practicing letters, to practicing words, to taking dictation of phrases, then sentences, and then, start the progression over again with composition: generating words, to phrases, to short sentences, to longer sentences. A dysgraphic child will not readily transfer without explicit practice. You will probably also have to coach him through applied tasks, cueing him repeatedly for letter formation/grip/pressure while he is writing (very) brief samples. If he reaches the point where he does it consistently in live writing (of whatever level) with cueing, then you can start fading the cues gradually, and then move to the next level of live writing, most likely starting over with cues.
He may reach the point where the cues become automatic, or he may, like polarbear's son, always have to cue himself consciously. Be prepared for anywhere from one extreme to the other. Mainly, don't feel like you have to stress yourself or your son about reaching a specific place on the range of handwriting automaticity, and do remember how many things are higher priorities (effective written and oral communication, happiness :)) than handwriting per se.
