Maybe they are just doing what they did before to make him in the real world, but since they do not have to re-create the city of Zanarkand in entire they can focus everything on him and thus make him permanent. OR
Tidus was a guy killed in the first attack of Sin attack that destroyed Zanarkand. Otherwise he would not have been re-created in the dream he would have been one of the fayth in the stone wall summoning that dream. Since his pyraflies still existed, they could have simply reformed him with the memories like someone else had mentioned and made him live once more. OR
I could be wrong. If I ever meet the write of the story line I'll ask, but probally he never came up with the explannation.