I see a number of things here. The diet being the first, that's a pretty hot diet and he's going to use that energy you're giving him. The next, he is getting quality feed, but does he have any supplements? I'd look into Magnesium for him: Magnesium- The Mineral Superhero
My next comment: are you using a snaffle bit that's connected to the hackamore? That's a bit much, I know this is a forward horse but those combination bridles are very confusing for horses who aren't completely 'finished'.
Personally I'd take him down to just snaffle (french link full cheek with keepers is Always my first choice) and I'd long-line him, walk+trot, over poles, through obstacles and even on trails if you have the opportunity. It sounds like his previous rider Only used him for speed riding, he may not understand that being ridden doesn't require going fast. Most horses are pleased when they realize that they don't need to Run all the time. After long lining him through an intellectually challenging course for him, things that make him think rather than just get him worked up (like lunging would). Then I'd hop on while he's being quiet and thoughtful. Then it just truly comes down to Practice! Practice using soft cues then slowly building them up to stronger cues. Remember (as I'm sure you do) to use you're entire body to calm him, think happy thoughts remember to BREATHE! Sing you're ABCs whenever you're feeling tense. Your attitude will effect him, if you're tense he will be bubbling up under you to go.
My last thought, you've only had him a week - has he just moved in? He may simply be extra hot because he is overwhelmed by the changes in his life and he may settle into life soon.
Good luck with your pony :)