At the first timeslips he had that suburban life with Ai, but later on, the timeslips were getting more and more locked until he gets to the "goes to Africa and dies" inexorable ending.
Supposing he changed the future only when he acted on info obtained from a TS, what were the ones he messed up to end with that future (before graduation)?
My guess is that he screwed up on the very first one when he told her to stay beside her dying dog, thus giving her the desire to become a vet.
The whole series then showed the future getting more and more deviant.
Is this a viable interpretation? What went wrong?