Horse bit querry - what bit to use?

i currently have a pony on loan, his head gear is a kincade cross noseband with a cambridge mouth slotted kimblewick. He is a very strong pony, and i have tried the reins on all the slots, the first 2 does not stop him and on the third one he pulls his head into his chest to avoid it. He also takes a few seconds to respond to the bit, but he does come back eventually and thats when he pulls his head in. He is great in walk but once asked to trot on he gets strong and this is when he takes time on coming back and slowing down, and again pulling his head into his chest. He used to be used for pony events but at the age of 15, he still has the speed and mind of a youngster. And looking into the many types of bits its a mindfield so any info gladly received