Use vocal praise when the dog looks at you with slack in the lead, reinforced if you wish, with a treat. Use a very small piece of some high-value food like cheese, hot dog, freeze-dried liver, whatever he loves, as long as its small. Your dog pulls you, you pull to redirect the dog, releasing the tension as soon as the dog has turned towards you, at which point you make sure he knows you think that’s the greatest thing he’s ever done. Be consistent in the redirect and use lots of praise.
The No-Pull harness does not keep the dog from pulling. It helps you convey that pulling is much less rewarding than not pulling. You should see some improvement pretty quickly. The time it takes to change a behavior greatly depends on how consistent you are. Well-exercised dogs learn faster than dogs with too much pent-up energy, but all dogs can learn to walk politely.
For very ‘reactive’ dogs, try practicing in the house where there are fewer distractions. Also, changing direction frequently keeps the dog thinking about you. Praise him for keeping up.