Just do what Lee said, for one, and gently refocus him on HIS stuff - his past, his thoughts and ideas etc. Be a gentle and interested listener. He will appreciate that.

People do not need to reciprocate everything in a relationship. If he is so open, you do not have to reciprocate. If he enjoys such conversations, fine, but let him do the talking part and you will nod and ask pointed questions from time to time.