How much give is there when you push on the tail? Would you be able to get a picture?
Some dragons hold a lot of fat in their tails, which would make them feel squishy.
However if he's lost bone density due to a lack of calcium absorption (improper uv exposure and/or calcium in the diet) then the bone could have been leeched enough that it's very thin, causing the squishy feeling.
How close is the powersun to his basking surface?
When is the last time it was replaced?