Has he always been reluctant to canter? Could be some discomfort issues, but could be just as you said, just plain being lazy.
You know, I don't hvae a lot of advice without seeing a video. Are yo perhaps leaning forward too much and him ending up just trotting faster? He would do that to try and keep his balance with the rider leaning forward. Also, leaning forward puts you in a weaker position to exert your authority if necessary.
All I can think of is when my friends really laid back horse wouldn't canter. And I keept legging him and legging him and of course, I ended up leaning too far forward, so finally, I just raised one hand up high as if I had a whip in my hand and said, "Yah!" and he took off! Not a pretty canter depart, but it woke him up. Now, if he is reluctant, I raise that hand and show it to him, or slap my thigh and itmakes a big noise. Of course, I also put leg on in the traditional cue for canter, and ask him to tip his nose to the inside. But , this was just a kind of last ditch whoopee to get him to break out of the pattern, and it worked!
He has since gotten much better about cantering, tho he canters very heavily and poorly balanced.