I'd use his name (or username if he's not given one), no problem different people will interpret differently.
Sometimes I'm abrupt because I'm travelling, busy, about to go into a booking, limited signal etc and being as quick as I can.
I once replied to a text message on a day off, it had been a long day (at uni followed by something else). The person thought I was rude because I simply said "No I'm not available today" or "No not possible".
I did explain I was sorry I was on a day off, tired, thought a quick response would help him.