There are 3 solutions to this problem
1. Getting out of the house is good but with a purpose. Go with him and walk briskly or jog if you are fit enough! Excercise produces endorphins, a 'feel good' hormone in the body and has been shown to work as an ant- depressant.
2. Depression is a serious issue, you cannot begin to help your partner until he acknowledges the problem and gets professional help. Get your partner to ask for referall to the Community Psychiatric Nurse (CPN)at their local GP's. They are great! go together and work as a team, they have loads of ideas to help you cope. Sympathy is not always the right option, sometimes it can be just finding a task they feel able to cope with that gets them through to the next day.
3. Depression is a funny thing - its very hard to be sympathetic when ostensibly there is nothing for the depressed person to be unhappy about. That's the same reason it is such a terrible thing. Imagine feeling so down and so helpless, without knowing why and without any explanation! Crap!
try and motivate him. get him out of the house, even when he doesn't want to go. Talk to him. Let him know that you know he's not happy, that you're there for him (tho i'm sure you've done all this already! But keep telling him!).
Ultimately, in my opinion, he's the one in control of the situation. I am prone to depression. But after many years i have "decided" not to let it ruin me. (admittedly easier for some). Your boyfriend needs to know that there's a choice he can make. And you will help him stick to it.