I could have written this word for word last year about my Ds! Literally identical, however we are now in year one and he's come on SO much where it wasn't even bought up at all in this year's parents evening (different teacher from the reception one)
I mentioned how difficult he found writing (he still won't do it at home with me at all) and the teacher looked surprised, and said no he's right on track, doing well and just needs to work on finger spaces. I was flabbergasted and she pulled his book and he is writing sentences!!! Yes they are a bit messy but I can fully read them!
So the advice I would give that seems to have worked for us this far is...
Don't push at home, don't use rewards and don't really mention it AT ALL (I didn't want to give ds a complex about it and make it a long term issue) so I literally backed off completely and let school work on it but nothing with me or at home.
Increase activities that build finger strength like playdoh, kinetic sand, sticker books etc
Leave pens/paper etc always accessable for them to see them out and about and decide to pick up themselves (Ds never did but eventually as he got more confident at school he started too!) We just left it on our coffee table in the living room.
Treat them to new art supplies/colouring books etc to see if they choose one they might like to do a few minutes of colouring - especially if you do it with them together and let them boss you around 😂
A mini whiteboard is really good for them to scribble on and wipe off (D's liked it because it's not permanent like paper is so he can just run it out and mess around with it) same with using a paintbrush and water outside to paint the fence or the patio etc
Do craft activities (it was hard to get D's interested) but we would combine it with going on a nature walk to collect pinecones etc and then decorate them when we got home, or doing Christmas or Halloween themed crafts together was met with less resistance lol!
Keep working on things they enjoy and are good at at home to build their confidence in general. If they like maths, or reading, or music etc encourage that as they will be working on writing at school and building their confidence in other areas (even if you don't think they need it) will help them feel positive in general.
Obviously I'm not an expert but it's super common for boys to take longer to get to grips with writing and so I would honestly take a complete back seat, even though I know the thought of that is terrifying because you don't want them to fall further behind and you want to help whilst it's not too late etc BUT I really do think it's a matter of giving them time to just mature physically and it will fall into place.
Last weekend D's told me they did a wanted poster for evil pea (from super tato the book) in school and he wanted to recreate that activity at home - I couldn't have dreamed he would want to or be able to do that a year ago.
Yesterday ds started drawing with the paper we leave out, whilst I was doing the dishes and he came running in, he had written me a note that said 'to mummy, your the best mummy in the world, I love you' again I couldn't even imagine him doing that before!
They're still so little so I'm sure it will come, they just need time and they can't naturally be good at everything.