Horse stretching front legs out when ridden...

[spotty_pony]

My boy has just started doing this when he is excited (out hunting usually). It is accompanied by his flanks trembling, sometimes tail swishing and stamping/kicking out/trembling hind legs. When he does this I push him forwards as it only really happens when stood still - I try to keep him on the move as much as possible.

Could this be Azoturia or Shivers or PSSM (he is part Appaloosa) or maybe something else?

Any thoughts much appreciated.