I understand where you're coming from. I once has dad pick up and it was unexpected. He didn't prompt payment at all, so I said to him " did mom send this weeks payment?" He was so embarrased! He went into his wallet and pulled out cash. LOL Paid me $5.00 to much because he didn't have the wrong change.
I agree that you shouldn't have to go without because they havne't planned well. But you also want to look like the nice guy, it's good for business.
If I were you I would call him and say really nicely "Mom didn't pay me before she went away, is there any way you could pop by today? " He probably thought you were already taken care of.