I don’t think it’s a matter of “opening up too soon” with his front foot/leg… it’s that he opens it too much. Look where his front foot is pointing - toward the left hand batter’s box; he should point it straight at the plate or preferably at the right hand batter’s box.
The other major issue is that he is not getting to full back leg extension - and is rotating the BACK leg too early. The cue should be to just keep the entire body as long as possible until the back leg gets to (or near) full extension. THEN the front foot lands and braces, pointing to the right hand batter’s box.
I agree with the leg lift - too high and the leg swings out; keep the foot under the knee and don’t hook it back - this will prevent swinging and will be easier to keep the front foot “closed” at landing.
I’d also like to see better rhythm with the hands; as the knee goes up the hands go up - the knee goes down, the hands go down.