I like him. I don't see anything that would keep me from buying him. While he doesn't have the bone mass in his legs to make me think that he would hold up to heavy roping very long, he is only 4 1/2.
In my experience (especially foundation bred quarter horses) they will continue to grow until they are around 7 years old. Mine always seem to grow up in height until about 3-4 yrs and then will continue to grow out width wise and really gain body mass until about 6-7 years old.
I also noticed in your video that he had a lot of tail swishing going on, almost looks like your gullet area is to wide and might be siting down on his withers to far (can you get at least 3 fingers in between his withers and saddle?). Might check your saddle fit to make sure he doesn't get sour on you.