If you want to be nice about it, you say "hey, I know you've been having a tough time lately, and i've been helping you with some of your work, but it's just too much for me to pile on. I can help sometimes but not always, and what you're going through you can get through it, and maybe if you do some work it would take your mind off of things. Also, if you need to talk and/or vent come to me or someone that you trust."
Or you can just dump the pile onto someone else or simply refuse to do his work or do your first and then do his and not stress yourself, when he has something to do. Or just tell him get it together while he's at work and go home to be in a mood.
Having a career that aligns with who you are will make you an energetic, positive, and makes your life outside of work better. Your friends and family will notice the difference between when you like what you do and when you don't.
