I think Liam is just deeply emotionally, socially, and mentally stunted from always having his hobbies, personality, and appearance mostly out of his hands. Even prior to fame, his parents seemed a bit stage parenty.
Maybe I'm just more disillusioned from studying psychology but I don't doubt that they diagnosed him with every "catch all diagnosis" like depression, autism, anxiety, and probably bipolar too. I'm not saying I don't believe in these mental illnesses or that diagnostic labels can sometimes aid people in understanding behavior but when you look into a lot of this stuff you quickly realize how little evidence there is of it's existence. Not to mention that diagnostic criteria has gotten less and less so more and more are diagnosed and put on pills for life long treatment = money. Especially adhd. No reliable form of brain scans to diagnose it, only arbitrary questionnaires of symptoms to fill out to but yeah let's prescribe adderal to every 8-year-old children.