OP, it's not quite clear to me from your post whether your issue is that you're fed up with having a husband who 'mopes about' and doesn't want to chat (which I do understand is a difficult thing to live with) or whether you're worried that it's not healthy for him to bottle things up.
I suffer from bouts of clinical depression and when I'm going through it, I don't like talking about it in detail to anyone except doctors and therapists who need to know the detail in order to treat me.
I know that's not the case for everyone, but one of the key things about depression (or any chronic illness) for me personally is that it's actually extremely boring. Being asked how you're feeling and having to say 'absolutely terrible' day after day to someone you love can feel really oppressive and it also makes me feel guilty because it's miserable for them as well, so I say 'yeah, not bad' or 'OK' because I just want to shut the conversation down really.
Not everyone is like this. Some people find it really helpful to talk about it honestly and openly and at length with family and friends about exactly how they're feeling when they're depressed. (And I think there are actually some people with depression can actually become quite obsessed with talking about it in a way that's probably as unhealthy as not talking about it at all.)
I'm afraid 'just moping about' as you put it is very much par for the course for someone who is deeply depressed. If he's been at work all day, he's probably exhausted from trying to appear chatty and fine with his colleagues. He probably doesn't have the energy or the headspace to keep up that pretence at home. It's not about you, honestly. I know it's hard to be around someone who is depressed and doesn't want to talk and it can seem hurtful - but it's not personal and he probably just doesn't have the capacity to engage right now.
The important thing is that he knows he can talk to you if he wants to, and that you won't try to 'fix' him if he does (eg if he says he's feeling really shit, don't say 'oh, but you've got so many things to be grateful for' or 'but aren't you looking forward to [whatever nice thing is happening]' or 'why are you worrying about something so trivial').
There's some good advice on the Mind website that might be useful: www.mind.org.uk/information-support/types-of-mental-health-problems/depression/for-friends-and-family/
I'm really sorry your husband and you are going through this - it sucks that he's got to wait so long for his GP appointment, but it's a good sign that he's acknowledged he needs to see a doctor and that the appointment's been made. I hope he gets whatever help he needs, and I hope things get easier for you as well.