Ania,
Another thing you might find helpful is writing contests. I tried and failed a few years ago. My DS was a bit interested in with 'winning money' angle, but I couldn't quite pull it off. I'm thinking that since your son already has experience with winning contests, he might be an easier sell. There are books full of listing of writing contests, or try Hoagie's Link http://www.hoagiesgifted.org/contests.htm

Smiles,
Trinity

Coaching available, at SchoolSuccessSolutions.com