I’m sorry to hear this, I really hope it wasn’t the schools SENCO that spoke like that to your grandson. If not, I would suggest contacting the school senco and asking what they can do to help his return. He sounds brave in agreeing to go back and this should be supported by the school as well as you.
Alternatively, you could contact his head of year or even the headteacher in your request for support. Is there someone he could check in with during the day? Could he plan to go somewhere quiet during breaks?
you sound like a wonderful Nan, and sorry he is struggling. Lots of support and encouragement and taking one day at a time.