I was a child of divorce and the whole situation was very difficult for me.
first of all I was secretly thrilled..This filled me with guilt, because every one else was so upset.
My parents fought all of the time. They had not been happy for years. My father was quick tempered and controlling, but I loved him, just not the way he acted.
I do not know your situation with your ex wife, with each of your children, but I will tel you, I had a lot of guilt about my personal feelings.
I felt like if I sided with my mother, I would hurt my fathers feelings, but if I sided with my father, I would hurt my mothers feelings. There should not have been "sides". but that is how they acted/
Their divorce was awful. Lots of fighting arguing, many times about our care and who was responsible for us. etc..
Also as I became a teen, I really wanted to spend time with my friends (just like every other teen), but I felt bad that this meant I did not want to hang out with my dad. It was a drag to hang out with my dads on weekend. Boring . .. These were times my friends were doing all of the fun stuff. Guilt, guilt, and more guilt for me.. .
My suggestion is for you to speak with your son. In person if possible.
Have a safe (for him) way for him to tell you truthfully whatever it is he wants or needs to say to you. Assure him you will not be mad, that you will believe him and that all you want is for the 2 of you to be free to speak openly with each other from now on.
Let him know you miss him. Let him know you want to be a part of his life and you miss him when he is not around. Let him know you are worried that something has happened that you are not aware of, but you want to know so you 2 can work it out.
Also let him know he is free to say anything to you.
Consider maybe inviting one of his friends over to the lake house.
Maybe sometimes just you and 1 child can spend time together at times.
Also my father always seemed to have a girlfriend around and then once he got married my step mom. These were all nice people, but I needed and wanted to sometimes, just be with my dad, like it used to be. More guilt on my part.
There is obviously something going on. if he will not meet with you, I think it is time to try family counseling for you and your children.. it helped open my fathers eyes.