
What have you tried so far?
Equipment and method aside, remember that dobes are very independent and environment-driven; they are people-people indoors and in low distraction environments but out and about, many want to be off and doing all the things! So persuading them to walk with you can be challenging, especially if pulling is already becoming a habit. It gets them where they want to go. So first and foremost, you need to make sure that whatever you're using to reward him (you're rewarding him, right?) is worth his attention. No kibble or shop bought treats here - I'm talking pieces of cooked meat and cheese. Make it worth his while to stick with you.
Remember as well that dog behaviour is driven by reinforcement. Whatever is reinforced continues; what is not, does not. That reinforcement is not necessarily just what you give him but it can be interesting smells, seeing another dog, or simply moving forward. So whatever method you use, make sure that pulling is not being reinforced.
When I'm lead training a dog, if that dog pulls, I stop, and encourage them to come back to me (I do not pull them back). I don't wait for them to turn round - I actively encourage them back. The second their attention is back on me, I move forwards again, and THEN I feed when they are back next to me and in position. I do not feed when they come back to me, only once we're on the move again. Moving again reinforces them coming back but feeding it can result in a yoyo effect where the dog learns to pull, to be called back and get a treat. By only feeding when they're in the right position, that's what gets reinforced, and feeding in position strengthens it further. Every few steps, then gradually spreading it out. If the dog springs off once they have their first treat, then I'll switch to going a step further each time we've set off again before they get that treat (so they're coming back, we're moving on again, and a little further before that treat on each repetition). That way they learn to hang around with you and the duration increases.
Both reward timings work, it just depends on the dog as to which I use.