My ds2 is nt and he really struggled with writing anything - including his own name at the same age. We used to do five minutes a night on it.
We started out with him writing over what I had written, then moved onto the join the dots thing, then finally to him having a go freehand. He used to write his name twice every afternoon (shortened version of his first name intially, then first name, then both first and surname) after school and then we practised writing a different letter of the alphabet five times as well. Every day I drew him a different smiley face for all his effort and he really looked forward to what I was going to draw next.
We also bought some dot to dot books and some of the pre-handwriting books you can get which focus on wave patterns and zig-zags etc rather than letter formation. The one had a gold star for every page he completed and at the time it gave him a real sense of achievement to achieve something, rather than focussing on what he couldn't do (ie write his letters properly).
One thing he really enjoyed was drawing letter monsters on his magna-doodle. He still does it now sometimes when he is bored. You just get them to draw any letter and it doesn't matter how rough it is as they then add to it to make it into a fun creature or monster. I found this helped him to get past his negative associations with writing.
It also helped to keep the sessions really short, fun and rewarding and not to do the same thing every night.
He was on an IEP and on the 'bottom table' for the whole of year 1, while he 'caught up' and he wasn't the only one - there were quite a few in the same situation - interestingly, all boys. By the end of year 1 he had caught up enough that he was moved from the bottom group into the middle and he is now (just beginning year 2) sitting comfortably in the middle of that group.
From speaking to the teachers, it seems a lot of boys seem to struggle at this age and having ASD does predispose some children to fine motor problems.
Ds1 is a different kettle of fish. He has ASD and both fine and gross motor problems. He is now in Year 4 and his writing is atrocious and he finds drawing, using a ruler and using scissors really hard as well. We have recently (on the advice of the inclusion team) bought him some stabilo smove pencils which have helped a lot with his grip. We were also advised to get him a writing slope and look at his sitting position as it all helps to create the right position for writing. We found that using a junior sit fit helps him to sit up and hold his upper body better, as he has a tendency to slump forwards due to poor core and upper body strength.
That said, as he is so negative about his writing and its getting in the way of his school work, the inclusion team are working towards getting him to type his work instead of writing by the end of year 4, so we are currently trying to teach him to touch type on their advice. We are still working on his writing skills though as I don't want him to slip backwards and feel that writing skills are still important and shouldn't be shelved completely in favour of typing.
Oo, just thought, another recommendation was painting on an easel - large brushstrokes. It doesn't even have to be a picture to start with, you can just get him to paint a whole sheet a single colour or maybe stripes or swirls. The bigger the arm movements the better to strengthen his upper back and shoulders. Also, a friend who is an OT said if you have any walls that need painting, give him a roller and paintbrush and set him to work - again excellent upper back and shoulder exercise.