Hmm. I'm a bit confused about what's going on here, but since DS9 also has always had trouble with handwriting, let me try...
His pre-algebra teacher presumably normally works with much older students, right? In that case I think her judgement of whether he has trouble that needs treatment (as opposed to accommodating) is not useful. If you put a child in an environment where the handwriting/fine motor expectations are those that are reasonable for older children, it's unsurprising that they struggle and that might show up as a problem with neatness, speed, or whatever. I think I'd sympathise with the teacher on the difficulty of finding ways for him to express what he can do given his asynchronicity, give her some ideas where possible, and ignore.
Compared with age mates, what problems does he have, exactly? Lack of neatness? But if he's writing faster than average, he may just be making that neatness/speed trade-off differently from some; maybe consciously thinking about the tradeoff might help? I might suggest picking a paragraph and writing it out repeatedly, at different speeds and neatnesses, to help get the feel of how you can choose a point on the spectrum. You mention spacing and letter formation - are those current problems? Trouble writing the right size to fit in a designated box strikes me as pretty typical at this age. Ditto using a method to tell left from right rather than just knowing (I still do, although you probably wouldn't know what the tiny hand movement I make is for if you watched me!)
The key thing seems to be that it's bothering him. That suggests something needs to be done, but whether the something is professional help or just helping him to find a way to work at it himself depends on whether you think there's really a problem beyond asynchronicity.
