L.B.
Whatever else you do, give him lots of extra love and get him involved in the important role of being big brother. There are some excellent tips on this, potty training, and new-baby regression, in particular, in one of the latter chapters of the Dr. Sears Discipline Book. Obviously you don't have much time to make it to the library these days. Thank goodness for amazon.com! I say, order it and read while nursing, during naptimes or those odd hours of the night when everyone else is asleep and you can't get yourself to wind down.
Good luck! Have fun!
L.