So, long story short, I met someone online and we hit it off. He was always very charming and playfully flirtatious and very supportive.

Fast forward, he came here to meet me and we hung out together for the duration of his stay, (a couple of days). Nothing sexual happened. We kissed, but that was it.

We discussed our feelings and both said that we liked each other, but ultimately decided to stay friends as he lives quite far away. It isn't far far away, but it's still a difficult distance
We also mutually agreed to continue talking afterwards. 

I know he has had a rough childhood and there's definitely something there in terms of a mental health issues, but I don't officially know what it is. 

Fast forward to now: He's been incredibly distant and we haven't spoken much.
He told me that he needs space, from everyone. 
Then he continues to be online on Snapchat, Facebook and WhatsApp, (he's on WhatsApp pretty much continuously).

I don't know what to do or where I stand with him. I know he really struggles with his mental health and all I want to do is support him and help him through whatever he's going through, because he would do it for me. 

He's acting almost non-responsive, leaving me on read, not returning compliments anymore and not sounding interested in me whatsoever. I can't figure out if he's choosing to discard me, or it's something that he's battling in his head and he's shutting me out to deal with it.
I suck at reading people and not knowing what the appropriate thing to do is makes a situation so much more stressful. 

It's so difficult to adjust to the relationship shifting so much, from it being so intense to pretty much nothing.
Why invest so much time, effort and support in each other just to drop it like it was nothing. 

What should I do? 
What do you guys think the reason for this is? 