He used to love broccoli florets, but suddenly he doesn't seem to be so impressed any more, and it's a shame...

It might be seasonal. My flock eats less vegetables in the fall/winter than they do in the spring and summer.

It could be. He certainly still likes treats though.

Today I wanted to give him a treat and when he saw it in my hand he jumped on it and would not even let me put it in the cage because he didn't want to get off it.

Birds always love treats.