This happened to mine shortly after I got him. He was around the same age as yours. I got so woried I talked to my old vet in the US(I live in Saudi Arabia and vets here are horrible, I can find out more from google).
But he had told me with one his age it could be a few things, as long as he doesn't seem in pain or irritated by the ear.
Could be a mix of a lop breed
Just a baby phase that he can outgrow
or if he is in extreme heat it can be caused by that. This was the case of my bunny( we had bought him from an outdoor bunny place and were transitioning him inside slowly). After a couple days in ac and cool temps, his ear popped right back up.