Just not interested

It's getting harder to get Jonty to go out for a walk nowadays, don't know if it's just his age as he will be 12 in July or just getting out of the habit.
Having grandchildren every day means he doesn't get the chance for a walk in the morning and by the afternoon he has just lost interest in the idea.
I could get up earlier and take him out but I'm getting older and more tired too!