Stifle Lock?

[PrettyPonie]

Today I was doing some inhand work with my two year old Riding Pony and when I backed him up to get him square I noticed that he was keeping his hind left leg slightly straight and wasn't going back normally wheras the other leg went back normally. He is fine when he is on the lunge and has a very big moving trot, is this stifle lock? And how much is there that I can do to fix this?
It only happens when he goes backwards or turns on haunches.

 

[TuscanBunnyGirl]

Does he ever do it when picking his feet up behind? My mare had this and it cleared up itself once she strengthened up behind It often happens when they have a growth spurt. Do you have a video of it?

 

[PrettyPonie]

I haven't noticed it when picking up his feet but will have a check tomorrow and take a video. I am hoping it is just due to a growth spurt as he is planned to be sold at the end of the season...