I can understand your concern and think that catching it early is the best answer. You are right to be concerned. He may or may not be falling into the grips of depression, but letting him know what is on your mind is the best you can (and should) do. But, being pushed by someone else to get help can make him clam up and even become hostile while he's suffering.
Personally, I like the hit-and-run approach. What I mean is, trying to engage him in a "serious talk" about his mental health isn't always productive. He probably pushes you away and gets incredibly frustrated. Start by coming up with a few bullet points to go over with him beforehand. For example:
1. statistics say depression is something he'll probably struggle with repeatedly over his lifetime
2. recent events in his life can be triggers for it to come back
3. he may not be depressed at the moment, but you have noticed a few changes in his habits that make you think he might be on the verge
4. you love him very much and don't want him to suffer
5. it might be a good idea to be proactive and see a shrink (even if it's without meds) and air out his feelings after a stressor occurs in his life but before he is deeply depressed... it's a really smart way to "manage" his recurring depression (personally, I see one anytime there's a stressor in my life that I feel is strong enough to bring it back... I'm a former sufferer and would rather catch it in the bud than deal with it once it is really ugly)
Ask him to sit down for just a second, that you have something you wanted to tell him. Then hit each of those points without glossing over your fears, without judging, without crying, without rambling, and without pushing for his agreement or input. Say you don't want to be pushy or put him on the spot. Ask him to think it over and that you're there for him if he wants to talk about anything. Tell him you love him, and then go do the dishes.
I think that letting him think it over himself is a great way to keep him from getting defensive or feeling cornered. I used to react that way when I was depressed, so I think I understand what you mean when you say that he isn't very approachable about the topic. It's important that he gets help if he is depressed, and someone else pointing it out can be helpful. If nothing else, knowing you care is important. Since what I said is true (he's more than likely going to suffer from again if he has once before), you will probably have this discussion again. Read about depression... educate yourself. Here is one book that may help you when trying to communicate with him: it's called
Talking to Depression (<---that's a link). Good luck... I hope things work out for you both.