I third. Lots and lots of work on a circle, getting him forward into the contact. My horse gets like this. She is very sensitive to machinery (starting gate?) sounds so making her so busy paying attention to me that she's forgotten all about the noises is very important.
But it's harder than it sounds, I know. Short reins, DONT PULL, and leg leg leg. Do not pull back, just keep a feel and Trot until the two of you are so bored with trotting you could just die. On a circle. The minute he puts his head up half halt and get him moving forward again.
Agreeing with Maddy, he needs extra stimulation or something he can channel his “funsies” into. On sites like Pinterest, you can find a ton of interesting flatwork or drills that will keep him on his toes.