Cast net recommendations for my son (14 years old, 5'6" tall)

[Mr T]

if your top priority if for him to LEARN to throw a cast net, go with the smaller versions, 4 or 5 ft. 6 ft too big for a first time thrower standing only 5.5ft tall!
'fold it in half' etc is not what you want to do to teach him how to throw, that comes later.
I say go with smaller, work your up to bigger for learning purposes.