Only he knows how his body feels. If he has done it before then he knows what he is getting into. Ensure is suppost to be really good energy source. I know it sounds gross, but a lot of ultra runners use it for quick energy, that does not upset the tummy. What about baby food, doesn't that have lots of nutrients and such? How is he feeling right now, he many get on the bike to start training and find that he does not have the energy anyway. All you can do is trust and support him (and worry on the side because you care so much)