Plumbing Questions? Ask a Plumber for Answers ASAP
This is an electric shower, so is cold feed only. The poor flow on a hot setting suggests a blockage, so you (or your son) should check the inlet filter. It wouldn't be too unusual for the builder's plumbing contractor to fail to flush the system properly, or for debris to find its way into your supply as a result of other work being carried out on the same estate.