You need a control collar. A prong collar, not a choke-style one; those are potentially dangerous to the animal, but the ones that LOOK scary, with the metal fingers on the inside, are actually much safer, and work by folding open when the dog pulls too hard, but can't cause choking injuries. My friend who is into obedience training and all that stuff calls it "power steering for your dog", and my other friend who has a 220lb English Mastiff- it's the ONLY way to walk him- it won't take long before your dog realizes that you have a lot more control, and the breed is smart enough that he should settle down a little.

Does the dog know how to sit, or ANY commands?
This is what I would do, and you can just practice in the yard until he figures it out:
Put the control collar on him.
Have him SIT. Get him used to the idea that when the pointy collar is on, it's YOUR time, YOU are in charge and he is in training mode. Any commands he knows, work through them. It will probably help to have some treats in your pocket.
Just keep him FOCUSED on you.
Try walking him around on a very short leash, or even holding by the collar itself, right by your side. When he is in the right place, and walking nicely, give him a treat and tell him "good HEEL". Get him used to the new training word "Heel" so that in the future you can tell him HEEL and he will come stand next to you.
If he gets too frisky, say HEEL. Wait a moment to gauge his reaction to the new command, and if he ignores you, give a single sharp tug on the collar. This is called a CORRECTION.
Stop walking, get him back next to you, tell him 'good heel' when he is quiet next to you, and just keep doing this over and over until he understands where he is supposed to be walking.
You have to keep on top of this and be consistent- don't let him wander off and smell things while he is wearing the training collar. MAke him wait for playtime until the collar is off.
Anyway... just a little advice on training. German breeds are VERY intelligent and can also be very stubborn. Trust me, I know; my Boxer mix is a pleasure to train because she's so smart, but if she just isn't in the mood to be serious, OMG what a pain she becomes!!!