Does he have a collar?  Maybe you could tuck a note into his collar that says,
"Please neuter me so I stop hanging around other people's houses, wishing for
what I can't have."

When we first moved here, neighbors across the orchard had an old deerhound
called Old Red.  His muzzle was all grey and a good deal of him pretty near
dragged the ground.  But when our poodle was in heat he would drag himself
over here and camp on our front porch for the duration.  He didn't even act
much interested or hang close the doors or anything, but did run off any
other interested parties.  After we had her bred and she had an unsuccessful
litter, we went ahead and had her fixed as much for his sake as for hers and
ours.

Jo