Oh I do sympathise. THis is a similar situation with my partner and his ex who feeds the kids all kinds of crap.
One of the problems you are facing is the fact that dad is telling him nonsense and you naturally want to redress the balance. Unfortunately, when parents do this, it does inevitably leave the child feeling unsettled and confused and not knowing who to believe. Putting his fingers in his ears is probably his way of saying "I don't want to know, I don't want to hear this". Understandably so. I don't think it is you, it is the situation.
However, by completely ignoring it I think you will fare much better. Since I came on the scene, I have persuaded my FH not to "call" their mother in front of the children; or in fact discuss any of the things their mother does or says. Slowly we are winning the battle.
Something else I did was to contact the NSPCC and get details of a local charity so that the children had a form of counselling. The Family Mediation Service may also be able to help.
http://www.nfm.org.uk/
I do wish you the best of luck.