It sounds like you need to start separating your dog’s curiosity from his obedience. When you take him outside he’s over-excited by the new stimulation, which isn’t a bad thing, but he’s learned to associate the act of walking with it. Try finding somewhere you can take him off the lead, a dog park or the woods (if you’re confident of catching him), or investing in a long-rein lead (if you’re not). Make sure you know where you’re going, and that it doesn’t take too long to walk there from home or where you park. Don’t try for more than ten minutes at the start; dogs get daunted just like people do. Then you need to force him to focus on you, and only on you, from the time you set out to the moment you arrive. This is the hard part, so you’ll need plenty of patience. Try keeping his lead short – you don’t want to be dragging him, but he shouldn’t be able to walk more than a few inches’ distance from your side. Keep a good handful of treats closed in your fist so you can pass it in front of his nose and get his attention, but don’t reward him with an actual treat unless he’s looking at you. Talk to him. When he starts to strain towards a distraction, give him a small tug (this should be a reminder, not a physical correction) and if it doesn’t break his fixation, simply stop walking. You need to wait out his curiosity, and only continue walking once he’s stopped pulling after the distraction. This might take a while, over a lot of walks, but he’ll catch on to the pattern sooner or later. Then when you finally reach where you’re going, make a fuss over him and let him off the lead to go be crazy for an hour.

The point is to create a clear distinction between walking as an activity that’s still bound by pack dynamic, with his attention on you, and when it’s appropriate to release his pent-up energy.