Canaries go through their annual molt around July, but birds kept inside are subjected to unnatural cycles due to artificial light and warmth. This plays havoc with their hormones and often they molt or partial molt out of season.
The main thing to watch is that there should not be any significant bald patches and that feathers grow back where they are lost, molting is a gradual process that takes 6 to 8 weeks.
Plucking in canaries is quite unusual but not impossible. It is usually due to irritants like mites/lice or skin disorders.
Thank you all for your advice - I will treat for mites as well to see if that helps. I also think he may be bored, I have been trying to find a female for him but always only find breeding pairs, no single females. I am quite sure if he had a mate he would be much happier.