Illness/Sickness: Swollen axolotl

I have a pootly axolotl who I've had for about 8 years now (and he was a year old when I got him). Yesterday I noticed his neck was really swollen, then when I looked at him properly so were his legs and body. He was fine the day before. I cleaned him out last last weekend. He has a light flow of oxygen into his tank, no live plants, sand substrate, a filter and plastic log to hide in. I've used the same products for his water for years (aquatic terrarium water conditioner) although the one used for the last 2 cleans was a new batch. I periodically feed him earth worms and blood worms, although none recently, I feed him JBL Novo Lotl XL sinking pellets designed for axolotls. When I checked the water all the parameters (pH etc) were the same as usual, around pH 7-8, no nitrile, nitrates were very slightly higher than normal at 20, KH and GH were high (as usual, always lived in a hard water area). Really worried about him and can't seem to find a specialist vet in my area. His brother died through constipation which is when I changed the substrate from gravel to sand, but that was a few years ago now. I've fridged him and he seems to be shedding his coat/slime. From what I've read it's recommended to give him isotonic baths, but not sure what to do for the best, don't want to make him worse. His neck/head area and legs are the most noticeable areas of swelling, his gills look short and his underside seems lighter than usual.

Thank you for any advice you can offer.

Thank you that's really helpful. Did a search and could only find one in Surrey and Oxford. Live in East London.