nikos - just our experience as an example for you - we have DS1, 5.8 years, ASD DX and currently in Y1. DS1 had huge issues with fine motor skills in reception, this time last year he was still grasping the pencil as a toddler would, and his teachers weren't even sure whether he was left handed or right handed because he'd constantly swap hands. He could write his name, very awkwardly, but that was about it (and he has a short name ) He couldn't use scissors at all.
Following DX he had 5 external OT sessions followed by weekly OT sessions within school (20 mins each, three times per week) which he has continued to attend through Y1. We also did a load of stuff at home, various finger-strengthening games, colouring and writing practice etc as others have outlined, tried to make it as much fun as possible. We used many of the suggestions highlighted above.
Just one year on he can form every letter correctly and neatly, is writing whole words very confidently, colours in very well (and enjoys colouring, which literally amazes me!) and can use scissors really well too. OK he is by no means the best handwriter in his class but he has come on so, so much.
So some additional input can really really make a difference. My best tip in terms of overcoming resistance to working with you is bribery . Haribos, stickers and treats to work towards (i.e. we did one sticker per session, even if the first few sessions last about half a minute, 7 stickers bought him a small treat). We did one session daily and soon enough it was just part of the daily routine and he started to quite enjoy it, particularly as his skills improved so dramatically.