Crate, start crate training asap. He may fuss, but let him fuss. Like
@Rits said the pen may too much for him. And do exactly what
@Rits said about training the crate. Over time he will put himself in there and use it to shut down.
Crate training is the best thing you can do for your pup and you.
I even have one in my car because Pepper is the worst in the car. Plus, it’s safe. We then remove it when we travel and use it in the home we stay in.
Look up some info on crate training and make it fun, add treats when he goes in, start using a word for him to go to his crate, and reward when he does it.