I'm a preschool teacher, and here are a few of my suggestions.
Go to Ikea and get him a cheap easel. (It's easier to write on a vertical surface than a flat one.)
Do lots of games and activities that use fine motor skills (like puzzles or legos).
Make it a family expectation that everyone in the family writes something on cards -- family birthdays, Valentine cards, thank yous, Grandma & Grandpa's anniversary card, etc. We have done that since our children were tiny, and now they write wonderful, heartfelt messages on all the cards we send to family.
Look into "Handwriting Without Tears," -- you can google it easily -- a company that specializes in helping kids learn better penmanship.