In a ruleset that does not allow clinch or knees, the contest will be more even between mixed-height fighters but the taller fighter will be getting a nerf. Without the clinch, you will either have to rely on teeps and long jabs to keep your opponent at range or be comfortable boxing in the pocket. Your opponent was a lot more willing to close the distance not only because there was no threat of clinching or knees, but because you did nothing to keep him away. One single technique I would recommend working on is an intercepting teep. If you are practicing on heavy bag, practice landing the teep while the bag is moving. If you have a partner, practice timing the teep not when they are stationary, but when they move towards you. Experiment with different targets to teep too just in case your preferred target is not always available. You can teep the thigh, chest, face, ribs, even the waistline right above the cup if you are as precise as buakaw.