Odfod. It's not about him being a burden. It's the fear of what will happen to him, how he will be treated when there is no one left who loves him and is able to care for him. Probably a secure home as he tries to attack his hallucinations and can accidentally hurt you. Have you not seen the news stories about care home staff abusing the residents? Who wants that life for their child?
So yes, some conditions are termination worthy if that child will have a shit life filled with pain or fear and neglect.
His family would never consider him a burden but he's a lot younger than all of us. How will he be treated by people who do, once we're dead and he has no one to advocate for him?
I'm sure you've thought about your kids future. What they will do, who they will love, where they will live, if they will have children of their own, what adventures will they have?
When deciding whether to have a child, you do consider their future. My relatives future is pretty bleak. His symptoms are unmanageable and so far no medication has been able to suppress or reduce them.
His future consists of whether his hallucinations will convince him to kill himself or someone else and whether the residential home he eventually ends up in will treat him kindly or neglect/abuse him due to his behaviour.
Would his mum have chosen this life for him if she had known? I doubt it.