Personally, I don't think this guy is disinterested, for one clear reason... the fact that he did not say no.
Now, just because a person may be shy or evasive it doesn't mean they aren't disinterested. By him agreeing to plans, it yields some interest. He may be busy sure, he may be shy, of course, but possibly not disinterested. He may be leaning in that direction, but for him to agree to plans, it means he's willing to give it a chance.
As for my theories, Carl pretty much sums it up. He may be any one of those things that Carl explained.
You brought up some good points. It is possible that he may be this way in a relationship, to which case you probably may not like. If he is quiet in the relationship, it may prove too difficult, especially if you're the one doing most of the talking and decisions. I get the feeling he's the type that follows the waves, and doesn't make them. I could be dead wrong however. Just a passing thought.
My advice, I personally think you should follow up until either he says "no" or until you lose all interest. Again, Carl pretty much sums up all the theories that may be surrounding this guy, take it into consideration.