Dear Mom,
your family consists of one daughter, and two grandsons. You're out of the country more than half the year. It's not like you're swimming in family, duty or social occasions, and it wouldn't kill you to be polite and say yes when the one grandson asks if you'd like to come to the circus show the entire school has been working on for weeks. If not, the least you could do is come up with a plausible excuse or white lie, instead of a
stupid offputting "Ah, no, no, I don't think so.... I'm an old lady, you know, and too tired to be out and about much". Especially after you've just accepted an invitation to lunch a few days earlier - but hey, that was to celebrate YOUR birthday, wasn't it.
The kid is 12, and big enough to figure it out for himself. So you could just be up front about it, and tell him that you're not interested in anybody but yourself, and couldn't give a hoot how he feels about it.
