You know the old adage "if it ain't broke, don't fix it?" I think this is a time where that could be used...seriously...if he does fine in a snaffle, he doesn't need anything more. The side pull is going to add more pressure to the side of his face, as well as the pressure on the mouth. If you want to experiment, perhaps you could try a bitless bridle, or a plain sidepull (one without a bit). But, if he does fine in the D-Ring Snaffle, stick with it, there is no reason to switch.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."