Firstly, in a work place it is not your responsibility to support people, or enhance their moods or care for their well-being, mental or otherwise. There are medical professionals available to assist with these problems. You are both there to do a job and, from your post, it doesn't sound like his work output is suffering as a result from this perceived change in attitude. So, from this perspective alone, I'd say there's nothing you should be doing aside from treating them professionally.
However, as someone else mentioned in their answer, to me this sounds like some pretty strong signs that the individual may be suffering from depression. Depression is a serious issue that affects a huge variety of people to varying degrees, but can have catastrophic outcomes. Since this individual seems to only take off their mask when around you, and you alone, it is an opportunity for you, should you wish to, to step in and offer some support and/or advice. Should you wish to go down this avenue, I'd recommend looking at n_plum's response as they have beautifully articulated the way in which to recognise if they are suffering from depression and the subsequent actions you can or should take.
Depending on the company policies and initiatives in place (my workplace has groups set up around Mental Well-being, for example), it may be that you can make them aware of that.
The main thing I'd like to add, without wanting to sound too accusatory or offensive to you, is that the stigma:
I am not sure if I should be worried about him, or if I should rather just let it go, >focus on work only, and ask him to stop behaving like a teenager.
that often surrounds those with depression is fairly ignorant and can be pretty destructive on the whole. For me, it's like expecting someone with a broken ankle to walk completely normally without any help/aid. Why should we expect someone with an injured mind to continue living without pain or difficulty?
Ultimately, whether or not you decide to take action with regards to this is entirely up to you as, provided their work isn't suffering, there's no professional obligation for you to do so. Should you wish to step in from a moral or compassion based perspective, then just please keep in mind that depression is extremely difficult to live with, for all people involved, so being supportive and offering sincere help is the best way to go. Just be aware that, depending on how close/involved you get with this person's depression (should it transpire they do, in fact, have depression), it can be quite a heavy load to carry.
EDIT - 2020/04/03
I've seen some people say it doesn't seem serious at the moment, primarily because the individual in question hasn't issued threats to harm themselves yet or other similar more obvious notes.
I, personally, went through a severe depression and social anxiety, starting at around 12 years of age and continuing until I was mid-20s. I've got more of a handle on it now, but it still effects me, every day.
I am very close with my parents and always have been, but I kept it from them from 12 years of age until about 22 years of age. In my mind I didn't want them to be disappointed in me, even though they wouldn't have been. I opened up to a few people in my secondary school, and I was mocked and ignored because it seemed silly to them. So, without saying anything and without anyone noticing, I cut myself extensively. Some people will say that's stupid or pathetic, and perhaps it is, but for me it was the only thing that gave me any sort of reprieve from the storm in my head. No one found out about those scars until I accidentally exposed some of them.
Additionally, I secretly prayed to die every single day. Who knows what I would have done in the end if I hadn't met someone, funnily enough from work, who was patient with me and pushed me to seek help. I still owe so much to that one person taking the time to notice and gently broach the subject until they were able to convince me to talk to a therapist.
