Well, in my experience, some of these have been working ok:
"...so are you saying that you feel (fill in the blank)? Would you tell me more about that?"
"I can understand why that made you sad (frustrated, lonley, etc.)"
"Thanks for telling me how you feel. That's important information".
P.S. it's a good thing that SO tells you how he feels. It's much harder to respond approprietly, much less validate, if you are being forced to guess how your partner feels.
