Celery isn't really a useful vegetable. It's got alot of water and little vit c. Try bell peppers especialy red. They are slightly expensive but they have tons of vit c. You'd only have to get him to eat a little chunk of it a day to make his minimum requirements. Another option would be a little extra fruit since they tend to like fruit better. Kiwis are good for vit c. Check out the charts:
Guinea Lynx :: Vitamin C Chart