Feeling tired after starting to date?

My son and his girlfriend started dating a few days ago. They were friends for about a month before they dated. She said she wanted space and wasn't ready for a relationship but now says she is. Early in the friendship there was a dispute between the two. My son said he had really liked this girl in the beginning but now he feels nothing and doesn't understand why. He wants it to work with her and says when they aren't together he misses her but when he's with her he feels hollow. He also said he feels tired all the time. She told him today she didn't want to scare him off but she has had this tired feeling since they started dating as well. I'm really trying not to put too much into this because they are both only 16, but it's my son's first "real" girlfriend and he seems to want it to work out. I don't know what to tell him the tiredness is that they feel other than perhaps they both have their guard up for fear of being hurt or maybe they are both mentally exhausted trying to make it work when it's not meant to be. My son keeps saying if things don't work out with her he doesn't want to live, which of course has me ready to pull all my hair out, lock him in his room and not let him out til he's mature enough to handle dating. Any ideas as to what the "tiredness" these two feel might be??? Thanks everyone!