At the end of the day what are you going to do if you find out he is lying? Cease to be his mate? Haul him over the coals? Lots of people feel pressured into doing this lads/lasses weekend away stuff, and say all sorts of fibs to get out of it because they don't want to upset their mates. Ask yourself this, if he is lying, he obviously has a strong reason not to want to go on the trip. Personally, i would leave it at that, dont go to the trouble of catching him out (if he is lying) and then in future it's up to you whether you invite him again