Instead of using jumping poles, invest in some cavaletti or simply nail Xs to the end of your poles. This way you can first of all vary the height and second of all they will not roll, which can end up in an accident.
Then I would suggest that instead of putting them one at a time, start to put them down so that for two steps in a row he is trotting over them, then three, then four. This will be gradual but to get maximum benefit from trotting poles the horse needs to go through them in a row sequence. Once he is used to trotting through the poles in a normal stride, you can slightly lengthen and shorten the distances between them to increase his range of motion as well. Good luck!