Aw babe, don't start doubting the relationship because of this just yet

It sounds like depression to me, and if you can convince him to see his doctor about it, he can get on medication to treat it. Depression is a disease like any other. When treated appropriatly, there's a great chance that he will feel better about life and he'll be happy. If you're happy with your current relationship, don't consider leaving him just because of this. Know that he confided A LOT in you when he told you that he's never been happy a day in his life.