If he is diagnosed as hairline fracture, then most probably it is fracture of lateral malleaolus which give away, if it is so then depending on the swelling we can apply below knee POP splint after swelling subsided below knee POP cast can be apply for 6-8 weeks as needed.
Need more information regarding your symptoms . Kindly share x-ray done if any.
Next Steps
Kindly reach out clinic or online consultation for clarification regarding your problem.