My twelve year old nephews idea of what to do this summer is to play video games all day and hang in his room.
You HAVE to inspire and encourage boys to work. Wanting and liking to work is a blessing or a curse depending on how a child is brought up in this world. You're responsible for that life long attitude right now at this very moldable teenage time.
The farm of course had the chores and jobs to do that helped instill that work ethic which is so lost now in our society. But you do have a job they can do that has a visual reward and skills to learn they will use for a life time.
Taking care of that yard. Now the boys first response will be to roll his eyes. In his mind this is not "real" work. But there are men making a living out there at this job and those are the very words you need to say to him. MEN, make sure you use the word MEN, make a lot of money at this job and he should be making some money of his own this year.
Do you see the difference in how you presented that? If you said, "you need to go take care of the lawn," you just lost!
But if you said, "you need to make some money this summer, do you think you're man enough to not just mow the lawn but take over the fertilizing and fixing the sprinklers and mowing? I can either pay a man to do it or you can take it over if you think you're man enough." See the difference.
Another trick here is to NOT teach him how to do it but tell him to get on line and study it, then come tell you how he will do the job and you'll decide if he knows enough to do it right.
You've put an interest and a goal in his mind with a financial reward. Then it is very important to make comments about how good the yard looks if he does a good job.