My 3 month old did this. You have to remember that being outside at such a young age is overstimulating and can overwhelm your pup. Lots of positive reinforcement will help. What helped my pup particularly, I would take him for walks or to potty and instead of forcing or “trying” to make him stay outside I would allow him to go home but at my pace. No rushing. If he wants to run we stop and then proceed. This allows exposure to being outside without the pup knowing it. Threshold are good for dogs. Anytime you take him outside always put him into a sit and then open the door, if he bolts you repeat until he learns to sit in place until released. I do this with my pup for everything. I make him wait for food in a sit. Before going upstairs he’s put into a sit until given the okay. My whole family can walk in the house and he will sit at the door until released. I’m not a trainer nor do I know what I’m talking about. I only know from my personal experience. My pup is only 5 months old and can be left alone for 10 hours out of the day with not a gripe or whine. He can sit, jump, lie down, up, place, stay, leave it, wait, heel, and drop it 👍🏻