Erin:  It depends on 2 things. 

1.   What do you plan on writing?

2.   How depressed is he?

Those 2 subjects will determine how effective your letter is to him.  If the letter is for you: to vent out your feelings, then by all means send it.  I know in my deep depression I would just put aside the letter and not read it.  In a lighter depression I would read any letter.  So I’m not sure where your dso is on the depression scale.  What do you think?