My 18 yr old Fox Trotter does the same thing. I don't know if cantering is naturally hard for gaited horses or if it is just that everyone focuses on their intermediate gaits and never really rides the canter. Probably both!
So I feel for ya!
I have been working on not letting my mare go hollow at faster gaits but it's hard because it seems to be her modus operandi. I have had my best luck cantering her up slight inclines because it shifts her weight back to her hindquarters.
Cantering her (which I do just for fun out on the trails) is kind of an interesting experience. She is either pacing, cantering, half of both, or sometimes I will get a lovely canter until I feel this big jarring motion on her back end, which feels like a car shifting into gears very roughly, which I'm sure is her cross cantering like your horse. So long as we don't go down and I get some canter strides that is a successful canter for her, lol!
I know my horse is traveling hollow at faster gaits because pacing is also a sign of hollowness. But holding her together is easier said than done.
At a walk she will actually collect a bit, but the faster we go, the hollower she travels.
Anyway, from what I understand this is a common thing for gaited horses. I don't think it necessarily means he is in pain somewhere. More like he doesn't know how to use his body at speed. Just my 2 cents anyway.