Fingerpainting is WONDERFUL for developing better control! Any kind of arts-n-crafts will help. Don't just consider that the issue is "writing better," it really isn't. The issue is developing the fine motor skills in the hands, with better hand-eye coordination. So play-doh, finger-play songs/games, cooking, lacing cards, puzzles, painting with anything, including sponges and brushes and sand and glue and mustard and fingerpaint. Chalk is a TOTALLY different medium and yet it involves exactly the same muscles and skills. Don't get stuck on pencil-and-paper. Markers and crayons and clay all give you the same end result. Also, don't get stuck on visible results. Rather than a goal of well-written-letters, think of the goal as being "strong hands that he has better control of" and so... let it be FUN! A 5-year-old will learn far better through play than structured writing practice.
One of my favorite books when I was teaching was called Process Art, which was about art-for-the-sake-of-creating, not with a goal of something for Mom and Dad to hang up on the fridge or for making the same project that all the other kids did.
Control comes after the muscles - and the attention span - are ready.