I think my neighbor (29, male!) has an eating disorder, what can I do? (x/post from relationships)

(I'm reposting this because I found this subreddit just now and thought you might be a huge help on this subject!)

Hi everyone. I (23, f) have developed feelings for my neighbor (29, m) who lives in the apartment above me (I live on the 8th floor). Our contact has been reduced to small talk since he got into a relationship a few years ago and since his break-up I've noticed how much weight he has lost and some other worrying behavior.

A little background information: He moved into "my" apartment house 4 years ago and looked incredibly healthy (and sexy!) - he was muscular but well fed, not chubby or overweight. We liked each other a lot right away, even had a sort of a date, but I had to leave the country for a few month, (business related) and he got into a relationship with a woman he had known for quite a while during that time. Since then our conversations were reduced to general small talk if we crossed paths in our apartment house. He seemed fine though and since I was busy anyways I never had hard feelings about that. Then came the break-up a few months ago and it hit him really hard. He rarely, if ever, left the apartment for weeks and looked terrible when he did. After a couple of weeks he seemed better though. He started jogging, which actually relieved me because he was doing something outgoing and active.

First, he was jogging every weekend, not too long after that daily. Now he even joggs twice a day, each time for hours. Sometimes I see him struggle to get up the stairs after almost 3 hours of jogging (at 7 AM!) and as soon as he gets back from work he heads off again.

He has lost a worrying amount of weight, he looks pale and bony and seems to be constantly exhausted.

Yesterday I really freaked out, because I found him sitting on the stairs somewhere around the 3rd floor, still in his jogging cloths, all pale, and actually struggling to stay conscious it seemed. When he saw me he almost panicked, seemed totally embarrassed and tried to get up as fast as he could. Even before I could say anything he said he was just getting up the stairs and that he had only made a small break because his knee hurt. I proposed that maybe he could take a break from all the running to let his body recover a bit. But he got all nervous, said that his body just needs to get used to jogging a bit better. He said how frustrating it was that his body doesn't seem to adapt very well but that he's going to go for a harder jogging route from now on to make better progress. He tried to walk up a few steps with me, but had to sit down after only a few steps again, which made him even more embarassed. It looked to me like he was having a hard time keeping his eyes open. (Can that be related to the jogging or might it just have been a lack of sleep?)

I noticed that his jogging pants are way too big for him now and that he had actually used a stapler to staple the excess width of the waistband together. I helped him get up and then helped him get to his apartment, but all I felt when I touched him was bones, bones, bones. :( I kept trying to tell him to take a break from running, but he just kept making excuses as to why he was so exhausted. He showed me he had put on ankle-weights to make the running harder and kept repeating that this was just an exception and that he's stronger and fitter than it might seem to me right now. He said: "I know I'm still quite a bit out of shape, but I'm really working on it!". He said in a few weeks, he'll be running up the stairs in no time. When I hugged him good bye I could feel his ribcage (I could even feel ribs on his back - is that even possible?) and his portruding spinal column.

I still have feelings for him and I'm worried sick. I stay up at night when I know he's still running and listen hard whenever someone walkes up the stairs to make sure he gets back safe and hasn't collapsed somewhere.

I have a few questions that I feel really unsure about. Maybe some of you know more about this, I appreciate any help I can get!

• What should I do in general?

• How should I handle running into him on the stairs?

• Is there anything I can do as a non-family member that involves calling a doctor (even if it's not an obvious emergency) when I'm worried about him?

TL;DR: I have a crush on a neighbor that has been jogging excessively and has become seriously underweight after his last break-up. What can I do?