My view is this - I need to be sure I can be in control and safe at all times, in all situations. Front OR back. In your case, I would work on him until he learns to be calm at the back, and as soon as he is... He can go to the front as much as he likes! I don't care where they like to be - I DO care if where they don't like to be results in behaviour that may escalate to being dangerous.
So, basically, I always ensure my horses can ride anywhere in a group, and ocne I know that, I let them go wherever :] What if another person was riding him but didn't know the way/wasn't confident in front?
In regards to cutting off other horses... Big no-no! Does he move sideways off your leg? If so, simply yeild him back the other way, strongly. If a horse is getting toey at the back, I make it halt. I wait until they stand relaxed, then I let them walk on a loose rein. If they jog, I halt again, but I always let them walk off on a loose rein - Giving them a chance to do the right thing before they are corrected.