Your 15 year old will have a great time in the teen program, and yes, let him walk around on his own. Just discuss curfews, safety, etc b/4. You should have him check out the teen message baords here at CruiseMates.com before his cruise.
All of the dolphin experiences are very reputable -- either should be great.
Yes, cabins are not huge but you're not going to be in them much.
My kids and I love Teppanaki on NCL ships -- Japanese steak house. Make the res asap once aboard -- very popular and FUN show the chefs put on!
Smooth sailing,
Luisa Frey
Family Cruise Editor