Just wait until he turns 6 months old then you can take him cruising.
No cruise line will accept children under 6months. to get the best out of YOUR cruise choose a line that has properly Qualified nannies and a night nursery such as P & O, Ocean Village, Cunard. These cater very well for young children and give you a great holiday too. They have food for babies and will provide your choice if you tell them in advance. They also have all the bits such as nappies available so you dont need to pack them. Most of all they give you peace of mind that your child is properly cared for while you are dining or in the theatre/Spa or nightclub till 2am !!
Think about it again in about eight years time.