You need to work on your position first, stay really straight (you can even be a little behind at the beginning), put a lot of weight on the back to slow him down.
Then you need to work a lot on close transitions (walk, trot, trot, walk...) and slow down your own movement (1 time seated, 2times up). Work on circles to reduce his strength.
You can make a lot of transitions within the gait to show him that there are different ways of trotting.
Work with calm musique so that he settle himself on the rhythm of the musique.
Finally, put some ground poles so that he settle himself his rhythm.
I hope i could help you :)
Half haults about every three or four strides, or whenever they start to speed up again. And really sit on your pockets of your breeches. I have found that makes it much easier to sit, is sitting as deep as you can. But for the sitting trot, half haults are your best friend!
Pull and release the outside rien
Slow your posting then sit if your posting fast with them there rhythm isn’t gonna slow down.
Have a good amount of contact with your outside rein and use half halts! You wanna make sure that you sit deeply in the saddle with weight in your heels and make sure to keep your shoulders back :)