Depressed boyfriend Watch
the past few months I would say I've been borderline depressed and found it hard talking about it with my girlfriend but here are some things I found really helped.
About 'how do you help' id say just start off with a chat. Although it might be hard for you to initiate that chat, he should be able to trust you if you're together and hopefully once it's started, it'll help him talk about it more in the future; remember a problem shared is a problem halved in many occasions. If that doesn't go well tho id think maybe try talking about your own emotions subtly and see if he responds to them in a similar way. I found this was an easier way for me to talk about my feelings bc we could relate and share our problems together.
About who to tell I would try and keep the problem between yourselves unless it is getting serious, as in causing damage - either mentally or physically - to him or you both. This is because he may feel almost embarrassed that people know his problems or that he can't trust you as his secret is now out.
I hope you understand and what I've said helps