YA Fantasy book about a child trying to save an old king in a fantasy kingdom

[Jamie]

I read this as a big dark blue hardcover book in the early 90's. It seemed old then so maybe published 70s or 80s? Maybe written for 10-12 year olds?
Protagonist was a child (boy, I think) who lived at the castle. The king was well-loved but very elderly and expected to die but I think there were some evildoers who needed to be sorted out first.
I only have vague memories of the plot, sorry.
Structure - pretty sure chapters were structured around visits from different groups of fantasy folk (gnomes, fairies) who would come to the castle to try to save the king. Toward the end I think the child had a fairy woman helping keep him safe.
Themes - maybe just my memory of it but felt like the book was a gentle intro to grief and loss for kids.
Random details
- most of the action takes place in the castle, the interior of one of the castle's rooms is describes as mother-of-pearl (I remember having to look this word up at the time)
- at some point the child is taken from the castle by carriage
- I think the king had a floor-length white beard in the cover illustration but it's possible I accidentally made that up

[Rainbowheart]

Long shot, Mio, My Son?