I don't think that at 60 pounds he's ready for a kitesurfing training kite like the HQ Fluxx, primarily because of the larger size and the fact that the kite is specifically designed for maximum pull. In a 5mph breeze it will probably face-plant him and drag him down the beach. Mom will not like that. A kite in 1.2 or 1.3 size will be fine as his first, and unless the wind is gusting up over 15mph should allow him to stay standing. Once he gets very comfortable with controlling the smaller kite and gets up to about 100 pounds you can consider something in the 1.5 to 1.7 meter size range. A 2.5 meter kite in a 15mph wind can face-plant a 170 pound muscular man if he's not careful and gets caught by an unexpected gust. A 5 meter kite is enough to lift that same guy off of the ground and put him down 50 feet away. Watch some videos and remember that in most cases the person in the video knows what they're doing, meaning years of experience.
Start him out slow and easy. He'll let you know when he's ready for bigger stuff. Personally, my recommendation for him is a Symphony 1.3 meter. And keep it under 10 mph the first few times out.
And get one for yourself in 1.5 meter. This way you can fly together and you'll have the next step ready to go.