a pirate boat comes in many different ways, from the traditional square rig galleon we all immediately think of, through Chinese junk type vessels used in Asia to the more modern small Somali type fishing boats with outboards or inboard motors as used in modern day piracy.
if you were to go with the latter two, being less complicated both in detail, rig and possible r/c, you could build or your son could with your help from artists card and board using simple knives, scissors and pva glues then waterproofed.
Jim.