Having trouble potty training my puppy.

This is my first dog that I need to train as an adult, so I'm pretty lost. My puppy is about 2 months old (I got him about 2 weeks ago from a friend) and I've got him mostly trained to pee outside with treats and reaffirming, but he's only ever pooped outside once. I take him out regularly and let him have plenty of time outside and I make sure to give him plenty of treats and love when he does go outside, but he still only poops indoors. When he does go inside I put him in time out in his cage for about 30 mins. I'm beginning to worry that this approach is only making him think pooping is bad in general and see the cage as bad rather than a safe space I'd like him to think it is.

Any advice from those more experienced would be greatly appreciated.