He should have challenged it.
I've seen that same play ruled as a TD in college and pros.
If the receiver cannot maintain possession by himself it is ruled a drop.
If the defender knocks the ball out after a catch and 2 feet down, I've seen it ruled a TD.
Even Trent Green said it looked like a TD.
Was McD saving his challenges for his challenge piggy bank ? You have challenges USE THEM !!!