I haven't dived Cocos or Truk but I think that starting with an easier liveaboard, probably in the Caribbean, would be a good idea. Just to try it out, find out if you both like the LOB venue, and to give your son more concentrated and recent dive experience. You can easily get in around 20-25 dives on many liveaboards - and it is a lot of fun, too!
The 2 you mentioned Belize and Cayman, are great but keep in mind that the Cayman Aggressor doesn't make the crossing to the sister islands about 25% of time depending on weather. If the weather is rough they will dive off of Grand Cayman and that may not be the "out at sea" experience you are looking for to try out the LOB lifestyle. In general, many of the Cayman Aggressor dives sites are also accessible from shore; the advantage is that you get to dive the best sites off all 3 Cayman islands - weather permitting.
I suggest taking a look at the Turks and Caicos, there are 2 popular liveaboards working the T&Cs and they offer frequent shark, turtle, and eagle ray encounters, lovely reefs, healthy fish populations, and easy access to the finest T&C diving IMO - because the best T&C reefs are a long boat ride from shore.
But you should check on recent reef conditions because the T&Cs (and many other Caribbean destinations) were hit hard by hurricanes this year - but I have been reading good things about T&C's reef recovery and rebuilding on Provo - so check it out!
But when are you planning to go? Both the T&C Explorer and the T&C Aggressor divert to the Silver Banks for whale watching from late January through the end of March. Right now the Aggressor is offering a 34% off sale on many of their LOBs which is a great deal, but you have to book very soon (by Nov 24!) and travel by the end of April, see the link below.
Have fun wherever you decide to go!
Scuba Diving Trips & Vacations on Liveaboard Boats | Aggressor Fleet