Most Helpful Guy
It took a lot for him to just tell you he is seeing a therapist. I just told my partner the same thing. He will tell you in his own time. In my case I'm being treated for trauma after being in an abusive marriage. So its not always a serious mental illness...
If he takes too long for your liking ask him, but try to be very empathic when you do it.
It does mean however he is very serious about you, because being treated for mental illness still has a huge stigma and to tell you that took a lot out of him.
Most Helpful Girl
If you're comfortable without knowing, I'd leave it. Just let him know you are there and ready to help and support him.