Thank you everyone for your responses.
I decided to take Cheddar to an exotics vet and was able to get an appointment for him earlier today.
The vet I saw said she could feel a mass and said an X-ray would determine what organs were affected (i.e. liver, spleen, kidneys, intestines), after which surgery may be an option. She said it was pushing on his diaphragm and would have been making it difficult for him to breathe
. At first she said the prognosis was poor but then after they put him under anaesthetic for the X-ray she was able to have a better feel of him and concluded that the problem was cysts on his organs, which they were able to drain while he was under (apparently they removed about 35ml of fluid).
After that she said the prognosis was much better and it would just be a case of keeping a close eye on him, monitoring his quality of life and taking him back to have the cysts drained again when necessary, as she said they will fill up again. She said they don’t know what causes this in older hams but it sounds like it’s manageable.
So hopefully he will be much more comfortable now and we will still have some time left with him yet