Here's a thought to consider - could he possibly be a bit of a visual-spatial learner, maybe with fine motor problems? The tip-offs are enjoying taking things apart and putting them back together, but being frustrated by legos, etc., and your post just above about the ideal school situation as having visual inputs and oral outputs (though lectures would be an auditory input).
In any event, it sounds like written output is the crux of the problem - do I have that right? And thinking out loud here, is it accurate to say that verbal output in an oral format is not a problem, whereas verbal output in a written format is? In other words, does he have the same "processing speed" issue with oral output as with written?
With a writing output problem, I'd definitely want to consider fine motor issues as well as a possible vision processing issue (though the fact that he's an excellent reader might point away from that). Has he been evaluated for either? Both of those problems may be forms of sensory processing issues (which is a vast world much larger than just tactile issues). OT can help fine motor, and keyboarding also seems to be the way to go, as far as school goes. For example of what can be done educationally, maybe a 504 plan that allows him to do his work on a computer. As for vision processing, there's vision therapy, which is often quite successful in fixing such a problem (my dd just finished this).
As for processing speed, supposedly it can be improved, though I think it's at least a little controversial as to whether the improvement is permanent. There are all kinds of therapies out there (I'm thinking along the lines of Interactive Metronome, etc.) though I really don't know any more about it, and I don't know if I'd be sure that was the problem.
Here are a few websites:
http://www.visualspatial.org/ (about visual-spatial learners - what one is, etc. plus helpful articles at the articles tab)
www.covd.org (about vision processing issues and how to find the right type of optometrist to do such an evaulation; not just any eye doc does this, and usually not opthamologists)
http://www.spdnetwork.org/aboutspd/defining.html (about sensory processing)
The other question I would have is what you mean exactly by learning holistically - do you mean learning whole before parts/details, as opposed to step by step (a visual spatial learner issue), or do you mean something else more general?
With written output taking so long, I think it would be very important to get to the bottom of whether it's truly some sort of general mental processing speed issue or whether it's a fine motor or vision issue, since those might well be treatable.
Just my two cents - I'll try to think more about it...
