Sorry to hear this news about your dad. It could well be ascites like Kris says and my dad had this as well. This is something that you should discuss with your dads doctors and here is a link about ascites that may be of interest to you -
http://www.macmillan.org.uk/Cancerinfor … cites.aspx
There are also loads of posts on the site about ascites and the search function will throw them up for you if you want to read more about that.
My best wishes to you and your dad,
Any advice or comments I give are based on personal experiences and knowledge and are my opinions only, they are not to be substituted for professional medical advice. Please seek professional advice from a qualified doctor or medical professional.