When I had to leash walk my pup at that age and even sooner I used a harness because she is such a puller and didn't want to choke her. Of course, that toasted my knee. I had to start using a pinch collar around 5 months, which is too young imo. Free running/playing is best. Have him chase you if you can run. Oh and I'd advise against chuck it or forced running until after 1 year old to reduce the liklihood of chronic/permanent injuries.