A harness won't stop him pulling, you'll need to re-train him to do that. Here's a youtube video that might be helpful for that: https://www.youtube.com/watch?v=ueE1S1k74Ao
But a harness will stop him from damaging himself so you can keep him safe while you retrain him. Many dogs - particularly energetic types like boxers - will pull on the lead hard enough to damage themselves, even if it does hurt a bit, the excitement of the moment overwhelms them. A dog pulling on a collar can damage his spine or thyroid gland.
I have personal experience of this : one of my lurchers used to be walked on a collar all the time: I didn't think it was a problem because he rarely pulled on the lead, was mostly walked offlead, and if he did pull it would just be a brief tug because he'd reacted to another dog barking at him, or something like that.
When he started having serious shoulder and back pains, I didn't make the connection. I rested him, he had pain relief, x-rays, nothing conclusive was found, but the problem kept recurring every so often.
It was only when I ended up with a spare harness and thought I'd walk him in that just to make use of it that I realised that walking him on a collar and having him very occasionally yank on it when he saw something exciting had been doing a lot of damage. The painful episodes stopped, he was able to walk further, seemed younger and clearly a happier dog.
I now walk all my dogs on harnesses when I'm using a lead. Now I think about it, I'd never attach a lead to a toddler's neck, that seems obviously risky, and they don't make seatbelts that go around people's necks either - yet some dogs could be going up to 30mph by the time they hit the end of a 6ft lead, so similar level of impact. My dogs do have collars, but only to hang their tags on, they are never used with leads.