Its wear and tear on your car too.. Just be polite and say that every now and then is fine, but do you (as in him) think that taking long trips is taking advantage, and can he see that? Also ask him whats up with his car, and if you can help to fix it etc?
Also, re insurance.. I have read somewhere, i think on the Honda s2000 forums, that some policys were now taking this off, so its not a granted. I would seriously get the guy to check, and depending on how much you really trust him, see proof too..
If he tools it, does he have the means to pay for it, and im not sure, but if he dmgs something/someone, you may be responsible too. He may turn around and just lie and say that he assume (or you said) that he was ok to drive it, he was insured too..
I have seen this happen before between 'friends'..