My 8 year old son has this too.
Every one of the three years so far of correspondence-course homeschooling, which already we only do a minimum of for assessment purposes, I have asked the "teacher"/corrector/marker of his papers to let him write in unjoined up cos joining up was impossible. My son said it didn't look like real writing, cos all real writing is in books and on computer, and is unjoined. Luckily the teachers have so far always agreed so long as is neat and otherwise well presented.
In the first year, after horror of having tried to join up for almost 2 terms, during which I often did a lot of it, writing with my left hand to simulate a six year old!! , to "restart" him we also used coloured felt-tips. He enjoyed choosing a different col for each new sentence etc.
He still doesn't like it much; it's slow and laborious, tho' neat .
What's so weird is that he loves drawing, and draws incredibly intricate, meticulous, fine detailed complex eye-views and maps, and "tech" style designs. In other words it's not a pure motricity problem, if at all!! It's as he didn't "understand" the purpose/function/reasoning behind letters, definitely when joined up, and that blocked him. What are these shapes? sort of reaction.
Since March he can read, so presumably that will change now, as realises can communicate with these odd shapes, despite their complete illogicality.
Perhaps he would have been happier with chinese/japanese logographic system.
Seriously do wonder about that. The Western alphabetic writing system based on sounds represented by symbols/visual codes for objects the SPOKEN words for which begin with that sound, a total muddle of the visual and the auditory, and the concrete and the abstract, may not be the easiest concept for an AS mind?
Maybe I should show him some chinese writing ?!