Tilly used to be the same way she actually used to throw fits. But I found just being consistent and pushing her she came through and now is doing incredible!
Sounds like you really need to get after this horse. Make a firm or scary sound with your voice. My favorite is "hey!" Or "get up there!" I've also had to take the reins with one hand and reach back and give a good smack. It usually only takes one or two times of that to get the message across if he's bright. Best of luck!