Baby standing in crib

So my LO is off the charts for height when we go to the doctor, he turned 8 months last week. He is becoming super mobile and loves going from sitting to standing. He has now unfortunately started doing it in the crib and will bounce, im getting afraid he will be able to flip out of the crib. We have the crib at the lowest setting and its kind of hard to see but on the bottom photo he can rest his arm and shoulder easily over the rails and is doing his bounce. If he coordinates pushing down on the rail and bounces he will go over. We are contemplating buying another matress to put on the floor next to the bed incase he does. Is there any suggestions on what to put there incase he does?