I could not see anything the matter with his back legs. Do you think he is lame?
I will say on your lounging, I find it a bit easier at the beginning of teaching to lounge to try and get more to the side of your horse, behind the center of his belly and "drive" him forward, sometimes to use a dressage whip, not lounge Whip. As an extension of your arm. You are making good use of your extra arm by pointing where you want him to go, but keep it up and pointing the whole time, so he gets to know the que. I just don't think your horse knows what you are asking yet.