Update

I went to see Mark again with Troy today. The corticosteroid has worked well and Troy has been eating well (and is also feeling much more comfortable with his joints too) over the last few days.

We discussed the options for Troy now, which are either continuing the steroids for Troy more or less permanently or to have him referred to do further diagnostict tests to find out what the underlying problem is. As Troy is 10 years old and the findings would most likely not be treatable, we have decided to go with the steroids. We will see in time whether we can reduce the dosage (though it's not a high dosage he is on anway 15mg/day) or whether he will need to stay on it permanently.

For now, Troy is doing really well and if we can give him another couple of years like this, then I am prepared to accept that deal.