In general he is quite a stubborn puppy
Please stop thinking of your puppy as "stubborn". dogs, especially puppies, are not stubborn - it just is not a thing with dogs. Attributing human characteristics to dogs is rarely helpful and often to the the detriment of the dog and the human-dog relationship.
If your dog (especially puppy!) doesn't do what you ask them to do, it is because:
--they don't yet fully understand what you want and the dog needs more training
--they are not motivated enough to do what you want and you need to find higher value rewards
--there is a medical or physical or emotional reason the dog doesn't want to do what you want, in which case medical evaluation (not in this case), more gentle training, and higher value treats combined with a great deal of patience, and 100% consistency, will overcome this in most cases.
In this case, just give it time and continue with the training.
Don't worry too much about his not going in on his own. If he doesn't, then don't give him a treat and just gently pop him in there, since that doesn't upset him. If he ever does go in on his own, give him bonus treats and a lot of praise. He will learn that if he refuses, he goes in anyway...but if he goes in on his own, he gets treats. Eventually he will go in on his own to get the treats.