If you share your own experience with depression you are not assuming how he feels you are just sharing your direct experience. He may be able to relate to it. Then you tell him that this is what you did to get help and manage. He might assume you are telling him to do the same which you kind of are but it is coming from someone who understands and is compassionate. He may get a little pissed but it will plant the seed and he will think about it. It can be a long process of realizing we need help but you are probably in the best position to encourage him. Of course don't nag him and constantly talk about it but be a safe and non judgemental person he can open up to. Having that person makes a big difference.