I bought two 500 ml soft bottles for the front pockets. I have found that I need about 1L of fluid (water or sport drink) for every hour I run. My pace is likely slower than your son (but pace on trails is tough to gauge). I also take gels about every 5 miles. If he does not get enough carbs and hydration on runs that are that length and longer, he will bonk.

Also, nipple tape and body glide is key to prevent chafing.