Welcome to the forum.
How long have you had him? There are a few things going on here, he is in a relatively new place, your new to him, and he is young.
If his previous owner used a rope halter then I would suggest you use a rope halter. It is not so much a matter of not liking the web halter, he does not respect it, a rope halter with knots applies pressure to points on their head and gives more control.
As for feeding him. Did you change his feed or keep him on the same diet his previous owner had him on? I don't see anything listed that would lead me to think it is his diet.
I think he is young horse with a lot of energy, in a new home trying out his new owner. I hope you have experience training young horses, though from your post it appears you do not. What he did is normal rude horse behavior, an experienced handler would understand that, and communicate to the horse that behavior is not acceptable.
Get a trainer or someone with experience with young horses to help you. Your right he could "flatten" you very easily, you need to learn how to handle him safely.