Some simple rules of thumb I was taught and are easy to teach to high school kids and their parents (They usually do not listen):
Balance: You need to eat and drink a balanced diet....as in you cant eat all carbs or all protein or drink all water. He needs to maintain strength and have energy!!!! That comes from multiple food sources and hydration.
Watch your sodium! Sodiums is good in terms of helping your body retain water, but you can over do it!! You have too much salt and that water won't burn off as easy. With liquids, water is number one and sports drinks or fancy waters should be 2nd. Balance is key!!!
Carbs to protein ratio: Breakfast/lunch he should try and eat 75% of his daily carbs and 25% of his daily proteins.....Dinner should be 25% carbs and 75% proteins. The theory here is he will burn off those carbs each day by general daily activities and obviously wrestling. He'll also be getting a good balance of protein to maintain his strengths.
Now this depends on how serious you/he is:
Good carbs: Sweet potatoes, red potatoes, rice, whole wheat breads are okay, but I would stay away from too much bread....
Proteins: Broiled or grilled Chicken, fish, lean beef, eggs (have you seen Big Tony), protein shakes--but you cant live off just protein shakes. You also need lean protein shakes, look at the sugars, fats and carbs in the shakes. High levels of BCAA's (branch chain amino acids) are good/better
Fruits/Vegetables: Greens are good and light and something that helps make you feel full!!! Broccoli, celery, kale. Apples and oranges are good and will add some natural sugars and hydration.
Treats: Everyone deserves to eat cake or pie or a candy bar now and again, so you don't need to rule those things out, just need to have priorities and limits.
He needs to learn that a snickers bar or a subway sandwich is a hell of a lot harder to burn off those calories than a pre-made dish of grilled chicken and rice. If you are on here asking questions, that tells me you want whats best for him. If thats the case you need to be of aide for him and make sure he has these food accessible at home all the time. You may need to cook or meal prep for him every week. Teach him how to do these things. Also may want to think about removing shitty snack foods from the house. You can't expect him to eat right all the time when he has a bag of doritios and coke staring at him.