Wait till yor child is three. This way he can enjoy the kids club included in your cruise price. Otherwise he will have to go to the baby club which is more like a day care and they charge $9 per hour.
You should definitely consider it. They do the best job with kids of any cruise I've been on. I have been on Celebrity, Royal Caribbean, Disney and Carnival. They also do a great job with the adults. They only thing they don't have is a casino.