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.
