I think I'd go over there and hand him a check for $120 with the words "FOR MOWING ONE TIME ONLY" on it and say some version of, 'Gee, I'm sorry for the misunderstanding. I thought I was doing you a favor. I can't afford to pay that much each month for mowing that small strip. If you insist, I'll have to make up the cost some other way. I happen to know a chicken/dairy/pig farmer who'll pay me to let him spread manure...'

Another approach would be more long-range. Sounds like he might not be able to keep the place once Mamma passes. How's her health? Are there siblings/other heirs? Would their acreage be farmable if you bought the place and bulldozed the house? Perhaps there could be some arrangement whereby you buy the place now (cash for them) and she (they) can stay as long as she can live there.

eta: I thought of another possible wrinkle. Have you ever met 'Mother'? Maybe it's not his idea to charge for the mowing. I can about hear the conversation. 'Whaddaya doing? I'm not buying fuel for you to mow some other guy's grass! He should be paying you!'