I'm guessing the problem is that no-one wants their lawns scalped during hot spells and, as you've allowed him to do the job in spite of the weather previously, he was relying on the income.
However, as a 'lawn cutter' he must be aware of this fact, so it's wrong of him to expect anyone to agree to it and, as others have pointed out- it's his choice to do this kind of work.
If on the previous occasion you only told him that you didn't want the grass cut at the last minute then, yes, he probably was a bit miffed.
I think it needs to be clarified with him whether he's expecting to be paid even when not needed for work in order to be 'retained'. These matters have to be agreed beforehand.