I agree working with your grandson's interests is a good way. My daughter was interested in the performing arts so I enrolled her in every performing art camp which helped her socially . We have something in NY called BOCES which has camps that go with the programs maybe you might have something like this in your state. You can reach out to the Autism Society they can tell you what's available for your grandson's age in your area.

I would concentrate on history, pets and one on one conversational and walks like you said he enjoys and develop him from these topics

He loves history, researching anything his has an interest in, loves animals , his cat, he'd love being outdoors but hates that we live in a city where there are always people. He's quite conversational ( although you do need to slip in there) with people but one on one, not crowds. If I could sell my place and move somewhere more remote, he'd go out. He used to go for walks or to the store when he lived in a small town in Maine

What does he like, what does he excel in

That is pretty much what happens to any child not diagnosed until adulthood. Says me it has the one advantage of allowing one to push against limitations one doesn't know one has, at the cost of reduced self esteem as one fails repeatedly at things that are easy for most people.