Well, first and foremost...spell check everything. Re-read everything you submit so you don't look like a fool to someone who is going to go out on a limb and risk their money.
Okay. Talk about what he has accomplished in his time in high school. Sports, volunteering, good grades, scouting, every non school related, and not lazy activity helps when you have nothing to start on.
Keep in mind, that when you have no experience, you are going to have the worst job an employer has to offer. Period. Next, if he's under age he may need to check with local laws to see if he needs paperwork filled out. Also, under age employees ( at least in my state ) don't handle registers or anything having to do with alcohol or tobacco sales.
Don't expect much when you first start out, but when you do finally get a job he needs to work very hard at what he does and have an interest at learning more. Employers have a hard time turning away young, energetic hard workers. ( Even if they have NO experience. )
When he goes to the interview ( and there will be one ) he needs to be cleaned up. Showered, teeth brushed, shaved hair cut some decent clothes. Your appearance is what will aid you in getting hired. Remember, if you look like crap, you must work like crap and employers go through a lot of crap, especially now that jobs are normally short in supply.
Be confident, listen to what your bosses have to say, work hard, learn and learn. When you finish a task, find whoever gave you the last task, and ask what is next. Do it the first time, every time.
I did this at the age of 16 in a Red Lobster as a buss boy, before I turned 17 I had moved up to dishwasher, through preparation cook and was cooking on the line. ( And had myself a nice raise at that. )
Life so far.