<human>: Could you distill the essence of the text into 1-3 sentences?

Hi!

I want to write a letter to someone I know going through basic training for the Marine Corps, however, I have no idea what to write! (For the record, he has asked me/posted online before he left for people to write him) 

I met him (Person A) a matter of years ago online, as he was a friend of my boyfriend's (Person B) at the time. Person A and Person B have not talked in at least a year, so there is no "mutual connection" anymore. 

When I first met Person A, he was a bit addled through minor drug use/drinking/partying/etc, however, he wanted to turn his life around by enlisting. This is one of the reasons I feel compelled to write, because I am proud of him making a good step. 

What is holding me back is the fact that he and I never were "truly" close, and he would flirt with me quite a bit - making the relationship a bit awkward. When we did talk, it was about a funny story he heard, or one of us going out with friends. (Personality wise, he acted a bit like a guy that one might meet on Tinder.) I wouldn't know what to say to him in a letter other than "I'm proud of you" which seems very generic. I've seen general advice saying to "talk about your life" but nothing new or noteworthy has happened with me, and I can't picture that being interesting to him? I told him I wanted to write him, and don't want to let him down.
<bot>: I want to support someone in basic by writing a letter, but I have no idea what to put because we were never truly close