Hello TahliaKate, thanks for posting your comment and sincerely wanting to help your friend in this awkward situation where you are caught between a rock and a hard place, knowing what you believe is the right action but unable to follow through.
His parents may already realise that something maybe wrong but too afraid to mention it, and being a parent I understand how this feels.
One of the main problems is that it's beginning to affect you as it's an enormous weight for you to carry and know that his parents would really want to help him, especially if he is struggling with depression, anxiety and bipolar, although I'm not a doctor.
Ask him if he thinks his parents already know by mentioning something he may do out of character and any reaction from his parents, that could be a way to make him realise they may know.
Look forward to hearing back from you at your own convenience.