He has learn that hacks are fun things where he gets to do what he wants to do. He must be taught that he can't get away with such behaviour.

Learn the art of half halts and use them to get him to stay on a light contact. At first you may have to really yank with one hand to get him off your hands, but if you do that every time he pulls he'll learn that it is more comfortable not to pull at all. 

You must not get into a pulling battle with him as he will win and you will have aching arms. In the arena, teach him to respond to your seat rather than your hands to stop and then use this whilst out hacking, only using your reins to half halt and remind him to listen to your seat. 

If you persevere he will start listening to you more, you must be firm and fair (rewarding him when he walks nicey) and stay calm and demand he listens to you rather than his friends.

My horse was a follower and used to bolt after the others ona hack if they disappeared round a corner. Her problems were confidence based, whereas yours are excitement (it sounds) but I think if you do what I used to do - get her to stand (just for a fraction of a second - she couldn't cope with anything more than that to start with), praise, then walk on - repeating this until he stops at your instruction and waits patiently for you to say go again, will help enormoulsy. And same with trot - wait for the others to go a few strides before asking him to go - don't let him shoot off as soon as they do - make him wait for you, even if that means yanking at his face - he has to learn to listen.