This guy is 20 and he needs to start acting like it. Showing once again that maturity does not come with age. Us men tend to take things way out of proportion especially in the initial "high" of a relationship. This guy sounds like he's taking it a little too far and needs to seriously back off some. Since you seem pretty convinced now that you're not gonna date him at least for now anyway I'd deffinetly let him know. Be straightforward and make it stick so there's no confusion and no mix ups. This guy is an adult by societal standards and he needs a reality check so don't be too soft on him just because you're confused and don't want to hurt him.
The "I only plan to date my future husband/wife" issue has been discussed a million times here before. The fact is very very few of us will actually do that. We can't limit God by making empty promises like that. The fact is God knew you would probably break that promise anyway. God may have some awesome wonderful Christian men later in your life that you may date. It's much better if we just leave things like that up to God and not try to micromanage it.