Generally a change like this means something very bad is going on in his personal life. Someone close to him may have died or been diagnosed with a serious disease, he might be going through a divorce or he might have been diagnosed with something. I bring up these things because that is the level of thing you may have to deal with if you choose to talk to him and he decides to confide. So be aware that you might be opening a can of worms you don't want to get into or are not prepared to handle if you ask about it.
I personally have asked coworkers about something that seems to be upsetting them and have had to help them get through a child with cancer, a spouse with cancer, a terminal diagnosis for themselves (she died about 8 months later and I was one of only four people at the office who knew of her disease and the only one who knew it was terminal.) and a very messy divorce. You take a real risk when you ask about this type of sadness that you will get more than you wanted to deal with. I have been through many of these types of things myself being widowed and having a great niece with cancer, etc. So I am prepared to deal with whatever a person might bring up. If you are not (as I was not in my twenties), it may be best to stay quiet and simply try to make sure that you don't add to this person's stress by creating work issues for them.
Generally when I see symptoms like this, I usually make sure that we are in a private place and say something along the lines of, "Are you OK? You seems very sad. Is there anything I can do to help?"
Now this accomplishes two things. First, it lets him know that people are noticing which he might not want if he doesn't want to tell and he can then be more careful about his behavior.
Next it lets him know that you are willing to help which can be important when someone is in a personal mess.
However, if he hasn't made a general announcement of a problem, he will likely say he is fine. That's ok, we don't owe a coworker a description of our personal problems. That doesn't mean it was wrong to ask if he was OK, even when they don't want to tell you the problem, people like to know others are concerned for them.
Just be aware that OK doesn't necessarily really mean OK. Look for ways you can relieve work stress on him if he continues to look sad. Sometimes the best way you can help is by not creating more stress. And don't continue to ask.
If he chooses to talk or breaks down, be prepared to handle that and understand that in general this type of conversation is confidential. Don't blab about it to others in the office. If he chooses to talk that is a big sign of being trusted. Be worthy of that trust.