I sometimes interact with a seriously autistic boy in my religious community, and watch closely how his parents work with him. You're on the right track with the holding. I'm sure you also preface your requests by calling his name.
The only other way I've found to (sometimes) establish a connection is to enter into his activity or stim for a few seconds. Not quick or easy, because it takes some study to figure out what his attention is going to at a given moment. It's pretty cool when it works, though. There's the actual possiblity of making a request or expressing affection for maybe a minute. His parents are always grateful for any other adults making the attempt to bridge the chasm into this young boy's isolation.
As you probably know, many kids on the autism spectrum have difficulty looking others in the face, because they don't understand facial expressions and so are confused by them. And many don't like certain kinds of touch. A light touch or soft stroke may be strongly irritating, even if a strong hug works well for them.
I'm so sorry to hear you and your grandson are facing this challenge, M.. I've noticed for awhile now that you give yourself generously to your family. It must be wearing, sometimes. I hope it's also deeply rewarding. I think your grandchildren are lucky to have you as their grandmother.