Has he got good insurance that covers him for all his medical conditions? Vital. Ask Thompsons if they are providing a wheelchair to assist him to board. Cruise lines usually do at port. Princess and P&O povide a wheelchair and crew member to assist those with breathing or walking difficulties, to board/disembark. Why not hire a folding wheelchair (red cross) to take with them, then all is covered.
Not sure about Liverpool but on board there are plenty of lifts and anyone who cannot manage the stairs will be allocated crew members to assist them in case of a emergency. Just make sure that reception are aware of the problem.