Spoilers.

He kind of screwed some stuff up in the 2nd half of the book. Stuff that scientists and sci-fi writers -- Star Trek and Back to the Future and Neal Stephenson come to mind -- have already laboriously pondered. Oh, and Plato.

The universe can't branch every time someone makes a decision. Law of conservation of energy. Matter. If there is an alternate universe where I am identical to myself except for this one thing, it's because that universe was created at the big bang, and because of probabilities when Infinity is involved, every atom and movement and chemical reaction in that universe, if it exists, happened exactly the same. But if I went back in time in this universe and changed something, it wouldn't affect the other universe unless someone in that other universe went back in their own time and did the same thing.

Basically, all the other Jasons trying to steal Jason's life are from other universes. All they have to do is accept that and go back into the box to find their own universes and leave the real one alone. But the author claims a new universe is created every time he does something so all the other Jasons are actually duplicates of himself, which is BS.