Changes in a person's ways of working and communicating may draw our attention and cause us to say "he/she is not doing well." The causes may be multiple and complex; it is better to avoid imagining or interpreting them. The question "is it your personal problems that are preventing you from concentrating?" may be, with good reason, very poorly perceived. It is likewise preferable to note what is different (what has changed) and to express it as a simple observation, without any attempt at analysis.       

It's not easy to find the balance point of distance/proximity at which one feels in one's place as a manager and also humanly benevolent. The co-worker may wish to share his problems and to be understood in his difficulty facing them. In this case it is a matter of listening, without judgement or advice. Listening without forgetting the context in which you listen -- that of a hierarchical relationship rather than a bond of friendship -- and so being attentive that the co-worker not go too far in sharing what may not be useful to us and which he might regret later.
Inversely, if he does not wish to share his worries, then you should respect his reserve. There's no need to know exactly what is happening, in order to support him.       

It is possible that personal problems have no impact upon performance, or that the co-worker does not want any adjustments or help. In the case where, on the contrary, adaptation is necessary, several questions: "In the current context, what could help you? How do you see your work at this time? How do you envision the coming weeks? What do you expect from me? ..."
Supporting a co-worker in his post is a discrete way of supporting him more globally.