sounds like he might not have the foundations sorted?
If he isn't confident on letters, I wouldn't be thinking about writing, or worksheets or anything like that - those are a few steps down the road.
For me with my son I followed this rough order:
1 - Learn letters (entirely through play)
2 - basic phonics, through play and cards
3 - basic reading, phonics books
4 - Starting writing/more "school like" work
5 - sight words
6 - more advanced books
For learning letters - a simple set of wooden blocks in letter shapes was great - can make them into a puzzle, play games with them, find certain ones, hide ones etc. My son's favorite was putting them all in a nonsense order, then I would read out the silly sounding nonsense word it made. That coupled with a load of A-Z alphabet books, and in no time they will know their letters.
For pen control, you say he already likes drawing + coloring which is great. If you want to encourage more intentional movements, you can buy or print simple mazes that they can trace like this:
https://toysandcrafts.co.uk/wp-content/uploads/2022/12/71UpcHCuW7L.AC_SL1500-1366x1260.jpg
Especially effective if you can get them about a theme your son likes (cars, space, dinosaurs etc.)
Also, when feel ready for him to write - wipe-clean writing books are great, because they are a bit more funny than paper, and he can wipe it off and keep trying, which makes it less frustrating, and great for practice.