Late responding but better late than never. He sounds like a perfect candidate for a Myler comfort style bit which I really encourage you to try since you already have it on hand. Fitted many horses and whenever I hear something along those lines, these bits on average tend to work best.
If you’ll go for a classic double jointed snaffle look out for something with a small diameter - preferably 10-12mm and a nice peanut style lozenge. Careful with rubber bits as they tend to be WAY too thick for that mouth conformation. If the tongue is fat I wouldn’t worry about the smaller surface area too much.
Based on what you’re saying we’d probably experiment with a loose ring, but Hunter Dee or a full cheek for extra lateral control might be worth a try.
Just a general note and observation: a lot of people tend to focus too much on the cheekpiece, I suspect because rulebooks can be quite prescriptive on them. But the mouthpiece is way more important for proper fit and acceptance. Unless the cheekpiece changes the rotation of the mouthpiece (ie Bauchers, curbs and gags rotate forward, not back so the action is somewhat different even for same mouthpiece), it mostly matters much less than the mouthpiece. Once you find a mouthpiece your horse accepts (no chomping, excessive pulling, hanging etc.) you can experiment with the cheekpiece to optimise the reactivity and lateral control.