Dyed Prince Frederick Wm 15 year of his age. Went into mourning 5 Jany.
Frederick William was the fifth son of the then Prince of Wales, Frederick Louis. Frederick and his father, George II, were at (metaphorical) daggers drawn (I'm not sure that is relevant, but I know it, so I'm telling you anyway).
This is the only portrait I can find of him - it is before he was breeched - Google doesn't like that spelling but it is to do with being put into breeches not being broken.