Re the water... will he drink diluted fruit juice - say 1 part fruit juice to 4 or 5 parts water? That way its still natural and doesnt have all of the extra sugar and colourings etc that cordial has.

Have you thought about maybe contacting someone from the Triple P parenting program for advice?

Has his behaviour changed dramatically since the birth of Paige? Because thats a big upheaval to his world. Is it possible to leave Paige with DH (or vice versa) and take him out for one on one time a couple of times a week or something?
I dont know how practical that is, as I dont have two kiddies yet...
Though I can understand some of your frustrations, as my son is the same age.
I agree consistency is the key as well... although it took me nearly a year to enforce the message to my son not to hit etc.

Goodluck sweetie, please keep us updated and feel free to vent in here if you need...