My son graduated high school in May 2020 and joined the Navy in August 2020. He is 18 and he lived with me full-time through July 2020. He and has the following financial situation: W-2 Income of $13473 (Navy plus PT job), a UTMA account in his name with Dividend and Capital Gain distributions, and he has contributed to both Traditional and Roth IRA's in his separate account (non-UTMA). Does he file his own return and does he report the UTMA distributions?
Yes, he should file his own return and report the UTMA distributions.
As an 18-year old active-duty military member for the period of time you described, it is unlikely he would qualify as your dependent (support test). For tax purposes, the UTMA account is treated as belonging to the child and your custodianship ended when he turned 18.
It is possible you provided more than 1/2 of his support for the entire year, but his earnings before he joined the military and his 4-months of active military service would argue otherwise. His UTMA have always been taxable to him.