His parents can't claim him. There are two types of dependents--qualifying child and qualifying relative. He had to be under age 24 at the end of 2018 and a full time student to be claimed as a qualifying child dependent. At age 24 he was too old to be a qualifying child dependent.
He had to make $4150 or less income in 2018 to be claimed as a qualifying relative dependent. Unless the "under 20k" income was $4150 or less, he had too much income to be claimed as a qualifying relative dependent.
Since you are married on 12/31/2018, you must file as married filing jointly or married filing separately. It is almost always better to file as married filing jointly.
