I encountered this issue with one of my old lease horses. 1 or two poles won't do, you need at least 4 spread out evenly and fairly close together. If he tried to jump the first, he'll quickly find out that there's another pole in the way and that it's easier to just pick his feet up and trot over rather than rocket launch over them!
Another option you could try is to space then out unevenly. I wouldn't do this consistently, but it may get the message across to your horse. He won't know what's coming next, so he will have to stay alert.
I suggest approaching the poles at a posting Trot, keeping a steady rhythm. Don't change anything before, during or after the line of poles. Just stay relaxed, and you horse will eventually respond to that!
Good luck! :)
I used to have a horse that had the same problem. I started a exercise that took me 1 month and some days until my horse did it perfectly. First I started with one pole and did it walking and then transitioned to trotting, every time he tries to jump a pole you stop in a straight line and walk backwards keep on doing it until he does it properly. Once done properly start adding poles to the sequence.