Hi! I believe I can be of help with this issue.
I can imagine how frustrating this situation must be for you. You are clearly a loving and caring grandparent and that's so important and wonderful. So the first rule is don't give up and don't become too frustrated. Remember that he will pick up on your frustration and it will make him even more frustrated than he already is. Be of good cheer and end every conversation you can on a happy note even if it was not that fulfilling for you and was brief.
The first rule with autism spectrum where there is no severe verbal impairment (like with Asperger's Syndrome) is to stay concrete. Don't ask questions that are too open ended right away. So don't ask, was the activity fun? That can be a minefield for his brain. Ask instead very concrete questions: was the activity outside or inside? You'll see that sometimes even asking: was the activity in a big room? Or, were a lot of people there? can be too open ended for some, maybe him. His brain might be stuck on what is a big room nd what is a small room?
So get to know his level of concreteness. Don't assume just because it's concrete to you and me that it's concrete to him. Learn his level. So if big/small, lot of people/a few people is too open ended for him, after waiting a minute and seeing that he got stuck, just ask right away a different question: what color was the room the activity was in? I do this all the time in session. It's very important to help him get out of being stuck by giving him a second chance with another question.
Then, avoid feeling words until you know what he can handle. So, no: did you like the activity? That's often too hard. Or, Was it fun? Yes, these are feeling words even though we're all so used to them. Stick with thinking words: Do you think it was a good activity? He may get stuck on "good" so learn that one too. Did you learn something when you were at the activity? A new word maybe?
Think, learn, do, those are concrete words. Feelings are tough.
And finally, help him out. If he's stuck, try to say it for him. But don't overdo it. If you ask did you learn something new at the activity? He may get stuck because it's too open ended what something new actually is. So you might jump in with, a new thing that you might have learned at this type of activity is how to grill chicken and meat. Grilling chicken is a good thing to know. Did you learn about grilling chicken?
Filling in the conversation for him is an important cuing technique. But the number one technique is still the one I said before of just asking another question when you see he's stuck so he can move on: did you learn a new song? Did something make you laugh at the activity?
Okay, I wish you the very best!
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