
One thing that might be worth doing - for the shorter term - is getting him used to wearing a muzzle. The baskerville type are best - those are the ones where they can open their mouths and pant - and they do design different shapes for different dog breeds. You would need to start by putting treats in it for him to eat out of it, so he gets used to putting his nose in it, then clipping it on, immediately unclipping and treating, and progressing to feeding treats with it on for gradually longer periods (few seconds at first) - you can put treats through it. The reason I suggest that is, with his size in particular, you run the risk of him hurting someone when he nips, or the risk of someone being scared and reporting him under the DDA, which you don't want. The muzzle will also make you feel more relaxed, as you will KNOW he can't nip, and the more relaxed you are, the more it will relax him - and you are right, a slack lead is better. :-)
I'll leave the behaviour advice for someone more experienced - like Lindsay - to come along, but I just thought that was worth a mention. :-)