If you're just dating, but not really interested in a relationship with him, he doesn't need to know.
The most that needs to be said if he suggests a grease-fest restaurant for a meal is that you've been trying to eat healthier lately and make an alternate suggestion (that still has the disgusting stuff he wants).
Most men you date are just that--a few dates. There's no reason for it to be mentioned to them at all. If things get more serious, then you discuss how important your current lifestyle is to you and why.