father/son relationship advice

Hi. Im looking for some advice on my 8 yr old son and his fathers relationship.
His father and i have been split for 4 and a half yrs. In this time i can count on one hand the amount of visits he has had and phone calls are far between, he will call every few days for a couple of weeks make a bunch of promises and then not call or anything for six months.
My son has behaviour issues that we have been working hard to help and over the last 6mths he has had massive improvements even the teachers have noticed. He wants/likes talking to his dad but i have noticed over these few years that he will have a good spell his behavour and then his dad rings and he goes backwards. Well on monday night his dad rang. He was fairly well behaved the next day which suprised me enough to point out to a friend. However 2 days after the phone call and he has been suspended for fighting. I have tried talking to his father about my concerns in the past and even went through mediation and pushed a parenting plan with regular visit/calls but he just makes excuses i threatened to cease all contact at the last visit if he did not keep regular contact but and he put up such a fuss about being the father and having rights but once again it was 6 months and my sons behaviour is getting worse when he does call.
So heres my problem i dont know whether to allow this to continue happening in the hope that one day he will grow up and be a responsible father. I know how important a father is in a childs life and i do not want to come in beween that. On the other hand the walking in and out of his life is obviously causing him alot of anger and confusion so is it best to cease the contact? I thought i had done the right thing by trying to keep the father/son relationship going but it just seems to have hurt him. Im sorry it isso long im just lost on ehat to do to help my son, any advice would be appreciated.