I'm a girl, i dont suffer from depression but typically, i have many ups and downs in my relationship. If he suffers from this depression then there is often nothing either of you can do about it.
The best way I guess is to just ignore it - as horrible as that sounds. Just carry on being happy and when he sees that you aren't bothered by the way he is acting then he may make more of a conscious effort as he sees that there is nothing he can do to bring you down too!
If, however, you feel down too, then there is nothing wrong with having a few arguments or serious conversations and even crying with each other. That may make him feel less inadequate.
Its not his fault, and just make sure that you talk and anything that may be bothering one of you, get it out in the open before it can esculate into something bigger.
Thats often the problem with anyone who suffers from this, as they bottle things up and often they all come out at once. Remind him you are there for him and you care about him whether he is up or down - But be prepared, its hard work for my boyfriend as it is, so I can imagine how difficult it must be to keep a straight face when you feel sad too.