Training my 11-year-old for first open water mile. Tips?

My son has decided that when we take our family trip to St. Croix so that I can do the lovely Coral Reef Swim (5 miles from Buck Island to the Bucaneer), he would like to do the 1-mile option. He's got a great stroke, is really strong, but has never done anything like this before. He's going to do the fins option, since it's his first time, and I am wondering about anyone who has trained their kid for something like this. He's comfortable in open water, and has grown up going to these swims with me, so he's not nervous about that part. My concern is making sure he can complete the distance. My husband is going to do the mile with him, so he won't be alone, and my husband will pull an open water buoy, just in case. It's from one beach, around a point, and to another beach, so there is an element of being somewhat off shore for a bit. Right now, he's swimming about 1300 yards a workout, with a combination of distances and intervals in there, and I'm upping it a bit each time. He is training every other day so he doesn't burn out. He's doing his training swims without fins, for the most part, so he can get accustomed to what that feels like. The swim is in one month so he will need to increase his yardage fairly quickly. I'm trying to go gradual so he doesn't end up hating it. He has NEVER expressed any interest in a competitive athletic event before so this is huge for him mentally. And he's very psyched for it. Any tips from other parents? Or coaches? Thanks!