Hmm, how about a "Do Not Feed" collar for a few days, just to get the point across. E-bay, about £2.
Tell the neighbours he's on a "special" diet, and the vet is monitoring his levels so he's not to have any titbits, at all, by law, ever.
OK - only a tiny white lie,
but it worked when Tufty was soliciting the neighbours here.
So pleased his results came back clear, but completely understand why you're still on pins.