Here is a lengthy interview where he discusses this at length (with a timestamp where he says exactly that String Theory, the precise mathematical model, doesn't describe the real world):
https://www.youtube.com/watch?v=2p_Hlm6aCok&t=10m15s
This also references this podcast discussion between Leonard Susskind and Lawrence Krauss, where they discuss the same thing:
https://www.youtube.com/watch?v=qhszd_wqAgQ
Note that he still thinks that there is a way to produce some kind of similar theory, "a string theory" as opposed to "String Theory", could be the best answer.
