Upvote:4
It took a little bit of searching but here is some information from the New York DMV.
Can I drive in New York State with an out-of-state learner permit?
You may drive in New York State with a learner permit issued in another state if
• you are age 16 or older
• your out-of-state learner permit is valid outside the issuing state
• you obey all restrictions of the state that issued the learner permit and (1) the general restrictions for learner permit holders in New York State and (2) if you are under age 18, the additional restrictions explained in the Graduated Driver License (GDL) Law and drivers under age 18
Meanwhile there are different restrictions based on where you are driving: upstate, NYC, or Long Island. The upstate restrictions are here. I consider it too long to quote here, but it sounds like he would be allowed to drive between 5am and 9pm as long as his grandmother has a valid license and is sitting in the front passenger seat. There can't be any under 21 passengers (except immediate family).
It's unclear to me whether nighttime driving would be legal. The supervising driver must be parent, guardian, or person "in loco parentis" and I'm not sure a grandparent counts for the last case.