SueR- You certainly can take him with to many of the on board activities. The baby sitting is an option but the is again in Camp Carnival not private sitting in the cabin. But for what its worth, the after hours sitting that they do offer is with other childern his own age and he may enjoy that.
But, the reason for my post is to make you aware of another alternative. On the longer cruises, Camp Carnival has a "all nighter" kids party. basically a big sleep over thing. They do charge for it and you could pick him up in th emiddle of the night if he doesn't want to do all night. But they has a blast (pizza making party, movie watching, games etc.) and you can have an evening to yourselves without having to worry about him. If it sounds like something you are interested in, go to the camp carnival "sign up" meeting on the first afternoon on board and sign him up for the party, yo ucan always cancel if you guys change you mind, but you might not be able to get in if the party fills up.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013