As @YElm explains, you should keep this for a private moment/email with this person.
Obviously, this depends on your pre-existing relationship and the character of the manager. If he's never shared much about his private life, you may want to refrain from saying anything.
If you decide to say something I would phrase things in another way in order to avoid coming across as nosy: rather than asking
'Are you OK?' or 'Is everything OK?'
I would simply sympathise and say something along those lines:
'I am glad you are back and I hope you are OK.'
This is a kind statement without explicitly asking the person to answer or add any detail. Be prepared to quickly go back to professional topic after this, so it does not feel akward if the person does not want to discuss it.