Boyfriend has depression. What do I do?

My boyfriend told me about 2 weeks ago that depression he has had in the past has now returned. I have no idea how to deal with it. We've only been together for 4 months. I've comforted him for things he feels bad about specifically but I really struggle to know what to say when he just says he's feeling low. I really care about him and feel terrible that he feels like this and I can't help, or that I just don't know the right thing to say :( the difference in his behaviour now that it's returned is sometimes quite obvious, but other times he seems fine... I just don't know what I can do. I know it sounds awful but as much as I care, I don't really like being around him when he feels so low, it's not nice to see it and I just feel awkward and helpless. I sound awful but I miss the 'old' him. And obviously I feel at least partially at fault for this happening. I can't explain why but I feel like this must be at least something to do with not being happy with me, me not being good enough or enough to keep him happy. I've already felt inferior to his ex girlfriend before and I can't help feeling even more so now... don't know if this is irrational or not. He says it's nothing to do with me. Advice anyone? :(

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