I was thinking sickle hocked too, but Im still learning so I didnt know if I was seeing things! Lol.
I think its the gravel that is making his hooves look different, and for his knee, it might be the angle or at that time we were having problems with him limping on that side, he had abcesses for a while. (Moved from a barn last august and I heard from different people that quite a few horses have been going lame from the place.) Even my friends two horses, and trainers horse came up lame for a bit- we all board at the same place, we all follow my trainer.