I went through something similar with my son's father. He has unlimited visitation with 24 hour notice....so he would tell me he was going to come visit on a Sunday afternoon. I would tell my son - he would bounce off the walls with excitement - Father would be a no show. I spent the rest of the day trying to console an upset toddler. I just stopped telling him when Dad was coming over - so every visit became a surprise for my son. It was easier and his little heart didn't break anymore.
Fast forward 12 years, my son is now 14, and really doesn't want anything to do with his Dad. He was in and out of my son's life so often that my son learned not to rely on him nor, honestly, believe anything he said. Dad blew off birthday 13 and Christmas 12 and 13 and that pretty much did it for my son.
BUT - through the years, I have spent copious amounts of time trying to facilitate a relationship between the two of them thinking that it would be better for my son to have a partially involved father than no father at all.
I was wrong. I stopped facilitating and let the burden of the relationship fall on the father. Kept an open line of communication with my son about his thoughts and feelings regarding his Dad and ensured that he realized that it was not his fault that Dad just was not a Dad. When he was 11 we visited a therapist to reinforce this.
At 14, he is well adjusted, understands that Dad is just not up to par. Doesn't feel to blame for it. Nor do I feel to blame.
Looking back I have often thought it all would have been easier had I just cut his father out of life completely - but, I refused to be the one to sever the chance of a father relationship for my son. As he got older he was able to see his Father as a man (and he does have some good qualities) and, with maturity, able to reconcile what he wants from a Father vs what his Father is able to give.
So let the visits continue - maybe don't tell your son in advance so he will not be disappointed when Dad doesn't show up. Teach your son that Dads are people and make mistakes and sometimes just can't be there. Teach him what a Father should be - my son has a completely different view of what a Father should be than what his Father is. Don't denigrate his Father to him - never say anything negative about him in front of your son...no matter how good it feels it is counterproductive.
It's a tough road, and sometimes a seemingly endless one - but with consistent love and conversations with you, your son will grow into a mature, well adjusted young man.
Good Luck and God Bless.