What to do with moody employee?

[Solenti]

I'm genuinely stuck and not sure where to go from here with one particular employee who works for me (own business).
He has always been outgoing and opinionated, he gets on with his work and is on time daily, is rather slow with tasks but does do them adequately. We are a small team, no bullying or issues, all very fair. My only issue is he is so moody! One day he can be ok, the next he will blank people, put ear plugs in (!), have a miserable/angry expression and it makes the atmosphere unbearable. Nobody knows what to say, we all try to act as normal and chat to him etc but it's so difficult as he is yes/no answers with a sullen expression. I've asked if things are ok, can I help etc but he says no. I can't think of any reason he would be unhappy with his role, he has flexible hours, can bring his son in if needed, free tea/coffee, frequent paid for days out/cakes etc, well paid. I don't know what more to do, but I need to do something as his moods are really frustrating for his colleagues and bring a terrible atmosphere. Any ideas?

I'm happy if he has days when he wants to be quiet and not chat etc...we all have those. I just don't want this snappy, moody, sulking

Youve summed it up here OP. You need to get this across in a chat.

Almost everyone in life has had something really awful happen at home and yet still manage to either put a brace face on for working hours or at least explain to colleagues that x has happened and might not be myself for a little while.

Being an arse to everyone yet not explaining what is wrong is unfair to the rest of the staff. Also, even the most moody of people can surely understand that whatever is bothering them isn't the fault of their employees.