I used zoloft (there is a generic for this, can't remember the name). I started out on Welbutrin, also. I don't know why, but at my first Onc appt., they switched me to zoloft. It did wonders for me. Also, I had a prescription for 1/2 mg. Ativan - 1 or 2 tablets every x (don't remember how often) as needed for anxiety.
I think the irritability is related to the steroids given during chemo. Chemo and radiation (either/both) can be making him tired and this may get worse as he receives more treatments.
My advice is to just let him sleep when he needs to and ignore his irritability as he really can't help it and, I'm sure, doesn't mean to say/do some of the things he is doing.
Muriel