Balleszabal Release t. 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)
Unfortunately, good and competent clubfitters in this country (Spain) shine for their absence.We have to go by trial and error.Release,one thing is for sure,he does not like high-kick, stiff tip shafts.
driver ss 106-110 mph swing. Quick tempo.Medium trajectory.It is for one of my sons.He does not like the feeling of high kick shafts
Unusual for someone whom has a quick tempo and very high swing speed that does not like tip stiff high kick driver shaft. Would it be 106-110 KPH instead of MPH ? Or perhaps it is a personal "feel" issue instead of the fitting issue here.
It matters which driver head he'll be using, with the modern design, a particular shaft will play and feel different from one head to the other.
Tinkering is fun, tinkering with proper tools will shorten the search for the near perfect combination for your son's choice. By comparing his favored driver for feel, as we know the best driver in the bag is not just the longest driver he could play but the "right feel" is important to put the tee shot in play under tournament pressure; as a starting point you could search for similar combination.
Or your son could head towards the general direction like most of the golfers , and "get used to " the new equipment.
If your son has not reached the physical maturity, or still working on his physical ability , then changing equipment at a higher frequency is expected. I know quite a few High School golf team members changes their bag each season by getting refitted to accommodate the growth of their height and strength.
Unfortunately, this also apply to a golfer going the other direction, as I'm finding game improvement features in golf equipment getting more and more attractive.