So, you will want to work on getting him to give to the bit and flex his head to the inside. You can do this standing, walking or trotting but I recommend walking. You can use an openning rein if you like but I tend to just use a normal (neither leading nor indirect) rein. I ask with a gentle pressure and when the horse gives in the jaw, you give release. To start with, be sure that you have the outside rein totally loose. You aren't trying to get him "on the bit" but rather get him soft to the inside rein.
As he starts giving to the inside rein, then instead of just rewarding for the give, ask him to also walk in a small circle, but he must stay bent to the inside and calm. Don't ask for a lot in the beginning and don't wear him out doing this incessantly.
Another thing that helps in this type of situation is to use leg yield. You get that bend to the inside, then using the inside leg , push him out to a bigger and bigger cirlcle. Use your inside leg in a pulse that is timed with each time he steps the inside leg under. You will feel his barrel roll to the outside when his inside leg reaches forward, ; that's the time to apply the "step over" aide.