He has only 12 hours under saddle so far?
First! With any horse that is new to being under saddle, take it slow and don't rush the process. Spend a lot of time working on the ground. Lunge him on schedule and be consistent in how much of a warm up you give him. Also, spend some time simply stepping up into the stirrup, dismounting, fully mounting, dismounting again. So on so forth until he is calm for being mounted.
Start off riding in smaller areas and practice some basic exercises like making circles both ways, stopping, backing up, etc.
Just be slow and patient, but also be firm in what you ask. If you press him on after an "episode" he will come to learn that it won't upset you or get him out of work. Seems like he just had a little scare, and if you approach him with confidence, he will calm down as well.