If I were you, I would want to talk to him. Depression is a serious condition, and no one should have to just stick it out and hope it gets better. There are so many resources available, I just don't see any reason not to try to address the issue.
When I look at having children, I view that as a stressor in life. It's a happy stressor, but a stressor nonetheless, and it's one that stays with you for a very long time. Stressors require attention, adapting your life, and some really difficult situations. When I consider any stressors that are coming up in my life, I try to have as few major ones to handle at once. I think depression is another stressor. It might not be a major issue for him right now, he might be able to manage everything in his life very well. However, it's really hard to predict how any person will adapt to adding a new stressor on top of that, even if it's something happy and exciting, and even if they have months to prepare. It's still a major change that takes a lot of physical, mental, and emotional energy.
In my opinion, his depression seems like something that can be dealt with now, instead of waiting to see how he will be affected by future events. Obviously it's totally your call, but if I were you, I would want to talk to him and encourage him to talk to a doctor to see if he can find a medication or therapy that helps.