a hobby loss is not self-assessed and is not automatic. It can be refuted if audited. Whether, if audited, the iRS would agree with you is a matter between your son and the IRS. if the IRS does conclude it's a hobby loss, he can appeal the decision.
see this thread from the iRS concerning factors used to differentiate between a hobby and business
https://www.irs.gov/newsroom/know-the-difference-between-a-hobby-and-a-business
looking at the IRS thread, I would conclude it's not a hobby. Remember, I'm not the iRS
