He's just starting to have problems with his joints. We're pretty sure that its arthritis so we need to keep his exercise to a gentle nature (although he needs more, he's gaining weight now). These backpacking trips can be long and arduous; sometimes up to 4 hrs. We try to pick times like this where there is plenty of water/snow throughout the hike so that he has ample opportunity to rest and cool down and this time he needed little hesitation to take breaks.
Anyway, Jaime and I decided that this would be his last one as we don't want to hasten any future problems. We'll probably keep taking him up, but it will be more along the lines of driving within a mile of the destination so he doesn't get so sore.