Leash training tips for large puppy?

I am around 125lbs and rescued a GSD/APBT mix as a very tiny puppy. Embark predicts that my pup will be 71lbs fully grown, but I’ve heard mixed feedback on the accuracy of their weight prediction. Even so, he is 18 weeks now clocking in at 30lbs, and I’m already having some difficulty getting him under control on the leash. He is not very engaged with me once we are outside, or even inside for that matter. We have briefly touched on some leash training basics, like rewarding him for checking in with me outside and impulse control. When he pulls me too hard, I stop until he looks back at me, then keep walking forward. He gets the idea at some point and stops pulling so much, but he starts pulling hard again eventually (I assume once he is tired, so I should probably end our walk at that point). If you have any tips on how to move forward they would be greatly appreciated!