I wouldn't push it, I would gently encourage 10 mins or so in an evening, and let them write in the Christmas cards etc....it doesn't matter if they get it wrong.
My 5 years old started school last year (Reception), he had been to private nursery prior to school, and as much as we tried to encourage him, he just wouldn't write, form letters/shapes or take any interest in phonics....he would actively move away from anything reading and writing related.
He started school in September and obviously we then had a mixed bag of school and home schooling. During that time there was a letter a day taught to the children to form, and a phonics sound.
My son moaned and resisted every time he had a new letter/sound to learn, and generally struggled to pick them up, we had years and tantrums daily.
We persevered with just a few minutes a day...no more, phonics sounds usually whilst doing something else...washing up etc, letters were practised on his wipe clean easel and he eventually got it.
He's in year 1 now and is reading confidently, and his writing is coming along nicely, however he still does occasionally spell his name wrong and we just correct him.
I would ask the school what phonics scheme they are using, our school uses Read, Write Inc, and they have a scheme learning a rhyme to form a letter, we found that helped our LO as some were funny and they had pictures within the letters to help remind him of how the letter is formed....maybe Google it to give you a better description than what I have!
Please don't worry, your LO is still only little, and it will come.