My younger brother has autism. He's 18, so a bit older than the OP's grandson, but we've successfully gotten him used to wearing the masks.
Where we live, masks have been required since April. There's very little room for exceptions; basically, if you can't wear one, you're supposed to request curbside service for essentials. So when virtual learning was going on still, the schools were talking about it with the kids, letting them know that masks were required, talking to them about the pandemic, why it was happening, etc.
Then we basically have my brother the option when things started to reopen: we told him he could either stay home all the time, or he could learn to wear the mask and go out. Gradually, he started learning to wear the mask. I started by taking him to an outdoor mall where there's food places, so we'd go inside, wear the masks while getting food and then take them off while eating. We gradually worked out way up to going on vacation in Hilton Head, where Disney requires that you wear them when not in a pool area or in your room. Now we go to Hersheypark basically every weekend. I never have any issue getting him to wear the mask; in fact yesterday, we were at the water park, and I came out of the changing area without a mask and immediately got called on it by him (there was no one around, but he was correct...)
The other thing we did was give him choices between the masks. We have Disney ones, we have disposable ones, we have homemade ones...his favorite is actually one we found at Walgreens. We think he likes the way the material feels.
It is a process, and this is a kid who doesn't like sunglasses or hats, but he understands the virus is here and it's what we have to do in order to have some fun.