Your OT will have good ides for you. In the meantime, this is what my OT is doing to help my son with his fine motor skills: forget tracing letters. He needs to learn to draw a vertical line and a horizontal line first. If he's got those down, move on to a circle and an X. Once he has those, then a triangle and a square. ONLY THEN should he start attempting letters.
I'm going to guess that most children heading into kindergarten will know how to write the alphabet, or part of it, but not many will be reading, so I'd focus on writing skills first.
In order to hold a writing utensil correctly (tripod hold - thumb, first finger, middle finger) instead of using a fist grip - break some crayons in half. He'll have to hold the pieces with three fingers because they will otherwise disappear into his fist.
I bought the "Handwriting Without Tears" workbook and audio CD (preschool set). My child is much more interested in listening to the music than actually doing any of the worksheets, but there are a couple in there that go with a catchy song about "the ant, the bug, and the bee" crawling up a wall - the worksheet shows the ant, bug, and bee on a brick wall with "lines" below them (presumably their walking path) and kiddo was willing to practice vertical lines. We're slowing edging into some of the others.
I have coloring books galore but he hates them. My OT suggested that a whole page of things to color is overwhelming, so we work on a tiny bit at a time: "Could you color that leaf? How about that one? Nicely done. Where is Christopher Robin's shoe? What color should his shoe be? Blue? OK - will you color his shoe? How about the other shoe?" If we get 3-4 minutes of concentrated work done, I'm happy.
Does your son string beads? It's a great fine motor skill if he needs that kind of help. Use string a different sizes of macaroni. If a string is too challenging, try pipe cleaners instead.
I'm excited to try some of the suggestions other parents have given you! There are so many great ideas here.