OTTB too excited when hacking

My new OTTB is 5yo and is getting restarted. We've been riding consistently together for a month. He's good in the ring and is generally calm and comes onto the bit, and is quite good at downward transitions. We do serpentines, WTC, and trotting poles. That's all good.

Where the trouble starts is when I take him hacking in a group. He gets very excited when trotting and going up/down ditch hills. He throws his head in the air when trotting, jigs at the walk, and just wants to go, leaps in and out of ditches. I try to be light with my hands and aids, but he'll just trot or canter off with me if I don't bring him back. Any ideas on how to calm him? He's smart and generally wants to please, but today we were at odds with one another.