A few things...
Heavy use of the machine, at least during some parts of the year, isn't uncommon at many colleges as well so he would be wise to figure out how to not hate it and make good use of it. Machines do have their place, at least for some aspects of hitting. Machines can have advantages as far as location consistency and good break if set properly, maintaining velo, etc. You can provide a steady dose of lefty and righty breaking balls with them. You can get unlimited bunting practice without wearing out the staff's live arms, and on and on. Depending on the capabilities of the staff, they can be anything from useful at times to an absolute necessity. And, ideally, a hitter will always have the attitude that he will kill any pitching, live or otherwise.
That said, your son is not alone in that opinion and, as 3and2 said, hitters definitely need to see live arms as well. Perhaps if there are others on the team besides just the one that already said something who feel the same, they can respectfully ask the coaching staff to mix more live with machine. Sometimes it is a timing issue as far as how the ball is fed. If this is the primary hangup, the players can ask for a more consistent "arm raise - then drop" type of feed so that it better simulates live.
I know I am over-simplifying the solutions - not easy for a soph hitter to address something like this with the HC, particularly if it has already come up and not received well. But there are strength in numbers, particularly when they include the 4 hitter. Also, the "mix" suggestion may be better received than something like "I can't hit off this machine". Lastly, your son probably didn't get to where he is at currently by relying on just team practices. Just shift some of his extra workouts to include cagework with a live arm.
Meanwhile, like every other challenge that is baseball, make the adjustment and have a positive approach.