If only we were that simple as a species.
The main clue, which you yourself have highlighted, is that it's a change in him - he's not normally like this. That is reason for concern and a sign something is likely going on in his world.
So, as you probably know, the way forward is to have a conversation - but what kind of conversation?
A careful and considered one, but ultimately one where you can express concern rather than provide discipline or consequences. You may yet need to do that, but don't do it first.
First is to approach gently, perhaps a quiet moment at work - if it is able to be private, or make a time to meet - towards the end of the day, or even offsite depending on your workplace.
But don't go to the pub - alcohol won't help.
Either way, the aim is to start by simply providing a "mirror" to him - to reflect back the facts about what you've noticed.
"I've noticed a change in you, you seem to be irritable and at times angry and it's not like you. I'm concerned and wanted to check if everything is okay in your world, and if there is anything we can do to help?"
It's probably a good idea to get clear about what help you can offer as an organisation first - time off, reduced hours, support to see a counsellor, or checking if your company has an employee assistance programme (EAP) that funds counselling.
And don't be surprised if he opens up to you, in which case don't panic, your job is just to be validating - "That sounds hard, no wonder you're upset" - and just listen. Resist the urge to fix it or give advice. Offer choices of how you can support him as an employee and let him decide.
On the other hand, he may stay closed down and say nothing's wrong. But it's important to have the conversation anyway, to let him know you see he's struggling and that you're there as his employer to help if he wants it.
Sometimes all we can do is plant the seed. And sometimes just knowing that someone cares is enough, even if to you it may not feel like it's very much at all.