Oh C., what a difficult position you are in! My parents divorced when I was VERY little, so all I know is my two separate families (both parents remarried within a year of the divorce, and are still married to those "new" partners). When I was about your son's age, I also did not want to see my father, and my father respectfully backed off. I didn't "have" to see him for one visit (Easter), but I do believe I had to go see him that summer.
Your son is old enough for several things to happen, depending on your exact relationship with your ex. First, I HIGHLY recommend that you continue pursuing counseling for your son. It doesn't have to be family counseling, as another mother said, he just needs someone impartial to listen to him, and perhaps to give him a few tools to cope with your divorce.
Secondly, if you can talk civily for any length of time with your ex, try asking him to back off, and to give your son some space. Even skipping one visit may make your son feel enough in control of his situation (because, let's face it, children of divorced parents have very little control over their relationships with each parent and visitation "laws") to be willing to see his dad next visit. Perhaps you could ask your ex if he would be willing to work a deal with your son (such as every other "scheduled" visit for six months, or half of the summer visitation...something like that?)
Third (especially if you can't/don't want to do the second), you might remind your son, that while he is growing up quickly, he IS still a child, and you are both still equally his parents, and you and your ex have firmly agreed to the terms of custody, and your son DOESN'T have any control (for now!). You might remind him that the custody situation IS temporary, as he (depending on the state of divorce) will be able to choose his own custody situation at X age (for me, the divorce was in TX and the age was 14). Just gently tell him that he's still a child in the eyes of the court, and the court has deemed this current custody situation to be best for him (and his sister!!), and when he is old enough, he'll be able to choose his own situation (and his desires will most likely change between now and then, you may find in 3-5 years he would rather live with his dad than you, because you may not know how to best raise a teenage son by yourself...which is absolutely NOT a slam to you and your parenting on my part... that's just the way HE may see it, when that time comes).
I DO have a psychologist that I am seeing (for other issues), and I'd be happpy to recommend her if you are near the Groton area... please feel free to PM me if you would like more info...I wouldn't mind chatting on the phone with you if you'd like...
I wish you The BEST of Luck... *hug* You'll all make it through this, and SOMEDAY, you'll have a wonderful, grownup son who does well for himself. :) Everything in childhood, good and bad, is only a phase.