The Eclipse is a fine shaft, but make sure it is spined/FLOed before installing. I use 'R', with a much slower swing than your son, in 1 of my drivers. It should give him the results he's looking for, with an 'S'. Another shaft to consider is the HOF Patriot. Look to Ebay for these, in 'S' flex. It might seem a bit 'loose' for him, but will keep the ball on line and with mid trajectory.
(re: your note that he plays 'S' flex)
Keeping in mind, not all shafts will perform the same way for all people, so recommendations can be very subjective. And a lot of those making recommendations are sometimes using the wrong flexes, in certain brands of shafts. I can use "A' in some brands, and "S' in some brands, with similar on course results. Always read the mfgrs. "design" statements, how their shafts were designed to perform; they should never be off base by much. Some test with humans, some with machines, so, keep in mind again, even their recommended flex to SS is still subjective, imo.