The major cruise lines do a good job of catering for those with special needs but you won't find much info on their websites when booking. It's best to contact a cruise specialist or call the Cruise line you are interested in for specific room information etc. There are a limited number of disabled access cabins on any ship so book early .
Try Royal Caribbean, they have the Independence of the Seas sailing from Southampton, so no flights involved. You can email the special needs dept for assistance. special_needs@rccl.com
Try to book a cruise with an itinerary that does not include tender ports- this means you have to be able to transfer with minimal assistance from the ship to a small boat in order to go into the port of call, which your son would not be able to do.
Some helpful info on this site too - http://www.cruisecritic.co.uk/articles.cfm?ID=105

Thanks emdia43. I did try this. I did not get a reply from Royal Caribbean, nor Gills, when I emailed about this. I guess they are busy before the new year. Will have to phone I presume.

OK most big ships will be suitable, some of the smaler ships will be OK as well but the older they are the more likely they will be less suitable.

What do I mean by suitable.

cabin, spacious, wide doors, wet room, often described as handycapped or HC on buliton boards
plenty of cabin grades with HC cabins

Access to nearly all decks by lifts(mosts ships have decks that have no lifts usualy upper sun decks)

Wheelchair freindly doors, most automatic and ramped or low sills. ..

Access to all entertainment areas especialy the show lounges

etc.

4 berth cabins are shown on the deck plans,
disabled cabins are shown onthe deck plans
You need to look for both symbols
Also pick HC cabins near a stair well if possible the stewards carts can block the coridors morning an evening.

on NCL these cabins are marked with a square block(HC) and a +(4) or *(5) berths
Once you decide what you want you will have to call, I woukld start with the cruise line for availability and then transfer the booking to the cheapest TA once you have the cabin on hold

Woops see you have now booked, Indepenence fine family choice.