Aries brother in law, depressed

Hi all, my Aries brother in law and I had a good long chat on christmas day. We used to be really close and were able to talk to each other and tell each other anything - but have drifted over the past few years with children coming along etc. His business went very bad a couple of years ago and he has worked extremely hard, 20 hour days x 7, to pull it back - its still a struggle for him but with his usual aries optimism he can see light at the end, and I believe he will truly be back at the top in the next couple of years. His marriage to my sister isn't the happiest but I had thought they were now on an even keel following a few bad years, until now. We can and do talk about everything so he spilled his feelings, he cried and told me how unhappy he/they were and how much he craves a passionate and physical relationship with someone who loves him and whom he can love back. He says I am the only one he has ever been able to talk to and I understand him, this is true, I do understand him very much - he does tend to get a lot of flak from others, mostly because they just don't understand his need to talk so in-depth about things, how he gets so passionate about subjects - also his passion comes off as anger sometimes but its really just him making his point in his arien way :-). I've promised to be there more for him and told him to ring me anytime he needs to talk, but I really feel his has sunk into a depression again (went through this with him about 6 years ago also). I can't tell or talk to my sister about him as she kindof scoffs at the way he is, she is also very much a head in the sand type of person - I will though bring it up with her. His family isn't the best and the only one he was semi-close to is his sister, who betrayed him badly a few years ago, his mother treated him very badly also a few years ago and even told him that he was no longer her son - they are ok'ish now though. Now he has no-one. I'm so worried about him.