Hi B.,
Could it be that the parents don't like to go? Parental socialization is quite a lot when you spend so much time alone at home. We are so homebodies here and if I don't talk someone to death, I don't usually talk at all. :)
If they do not try yet it is okay, careful not to be pushy. You don't want them to be mad at you then not listen to you when something is really important.
Kids play by themselves at that age quite a bit, especially boys. I can still put my son in a room of kids and he will find something to occupy himself with all the chaos and laughter around him. They just like to think and explore and build. Now at six, he is way more social.
The only thing I worry about is this, if mom doesn't get out, she will be more dependent on her husband for her own happiness. It would be healthy for her to have a little social life on her own. That and...well..kids who spend a lot of time by themselves can get a little nerdy. :)
Does she play with him a lot at home? That will make all the difference.
And maybe...they are happy the way things are.
How sweet of you to offer a playtime for your grandson. How generous of you. Keep up the good work!
God bless, C.