Adult stepsons mental health issues is testing our marriage

When I married my husband his son was in his early teens. At that stage things on the whole appeared to be OK although his son spent a lot of time playing computer games and was socially very awkward. When he was in late teens his anxiety worsened and he started to see a psychologist. His social anxiety escalated after a relationship breakup two years ago he has become more isolated and non communicative. Now in his mid 20s, he spends most of the day in his room on his computer. He has been diagnosed with a personality disorder and depression and spent time in a mental health facility. He makes no effort to get a job or study in spite of encouragement and support from my husband and I. We understand he has been prescribed medication but we don't know if he is taking it as he only gets angry when we try to communicate with him. We also have a younger son living at home who is also impacted by his older step brother. My husband refuses to discuss his son and whenever I try to raise the topic we end up fighting. I have asked my husband to set some boundaries for our stepson, ie getting out of bed at reasonable time of day, eating meals with the family, helping out with chores around the house but he does not want to deal with it. I know he feels awful that his ex wife abandoned his son when he was young but that was almost 15 years ago. I am becoming more and more frustrated with the situation and cannot see how we can find a solution.