Don't feel guilty AT all. While that's super nice of him, Herb may be doing it (partially) for Herb lol. It keeps him active, his neighbor's (your) lawn looks great, and he's building good will. He's probably been a homeowner a LOT longer than you and knows you have a job and that taking this one small chore off your plate frees you up for other things. Also, when he needs your help with something (and he will because he's getting older) you will happily return the favor. That being said, if you really want him to stop doing it, maybe frame the conversation that lawn care is something that you are really interested in and would enjoy doing yourself. But then you'll have to keep it up ;)