I would say that next time he calls you ask him why you should do it? Or better yet what will he do for you first.
Here's the deal. If he weren't your son you wouldn't feel this way. You would know he is an adult choosing not to care for himself and you would walk away.
Make a deal with SO. If your child were a easy child what would you be willing to spend monthly on them. IE: grown child with a job and no need for parents support.
I decided long ago around $30 to $50 dollars in lunches and coffees and maybe even mani pedis was where I was comfortable. If that works for you and SO then be willing to give that amount in support without condemnation. Over that amount you discuss.
I have gone so far as to ensure the money doesn't go elsewhere by buying what I think is important only. Buying gift cards to places they can only get food from. Walmart thought I was crazy for wanting a way to get a gift card they couldn't buy beer or cigs with. Heck I have even paid a bill so they could keep their cash for the item they think I should help with.
It's not foolproof. But reality is you love your kid and it's impossible to forget them or not care. It isn't impossible to find acceptable ways to be supportive without being manipulated.
My thought would be to ask the grannie a way to help without giving him cash or food.
