First off, I am a 69 year old coach and trainer. I was smart enought to realize there comes a time when you need NOT to box. As far as your diet, I dropped from 230lbs to 210 in five weeks to maintain my muscle mass, strength, stamina and move quicker. Without writing a book, here are a few tips: Eat a lot of vegetables, avoid all dairy products, bread, and anything white. SUGAR IS A BIG NO. Nothing fried. Fish, chicken, steak, broiled. I eat tuna and Sardines out of the can. Melba toast limited to 5 regular a day. I drink water, black coffee, and unsweet tea. For two weeks I was on a 500 calore diet a day..Yes...500 calories only..it was damned tough, but on the third week, the weight just fell off fast. I dreamed at night of eating..thats how tough it was. Absolutly nothing out of a vending machine or anything 'artificial" . Stay the hell away from fast food places. Sweet potatoes are good. third week, I had two eggs in the morning with coffee (eggs and anything cooked or basted with coconut oil only). See if you can find a diet plan called MSM or MSN and go by it. Good luck...takes a lot of discipline. I also kept up my workouts. For snacks...apples, oranges, grapefruit. Six very small meals a day and nothing after 8PM. Drink lots of water. This is not a comprehensive diet...just some ideas....Low fat peanut butter on apple slices is a great quick energy snack. (In moderation). Now if you make it...the real hard part is keeping it off. I have done so for the past four months.
My last advice....why do you want to fight at your age..what will you prove? You risk some serious injury and you will find even minor injuries are slow to heal. Do some light sparring with someone your own age. I work with amateurs, and know even a 14 year old kid can do damage.