Greg has admitted his mistakes and asked for Chris to forgive him. He has basically told Chris he's the only man in the world for him and he's willing to do anything to gain his trust and love. Chris recently noticed how Greg is the one to try and mend matters when they have a disagreement, no matter who is at fault. To be sure, they still have a long way to go before they can have a romantic relationship, and Greg has to show more of the original man Chris met and was attracted to.
Let's not forget that in the six years gone by, Greg only visited his father a few times, so the two main interactions of Greg and Chris were the original meeting and the disaster right after Greg's father died. Chris may recall Greg being nasty during those six years, but he'll soon have many months of Greg trying to win his regard to set against it.
And there was one huge advantage to Greg turning his back on Chris six years ago. Chris has had six wonderful years with Mr. Montgomery, maturing and becoming a man who can stand up to Greg and hold his own. They would not have had an equal relationship back then, but they may be able to now. Greg has suffered a lot more anguish during those six years than Chris, which serves him right, and he'll always have to live with the fact his mistake cost him his father's approval.
I don't know much about D/s relationships, but as far as I understand the sub has as much control as the Dom, just in a different way. I think that's why Greg laughed at the end, because in fact Chris was very much in control at the moment he decided to forgive Greg. What is the saying: To err is human, to forgive divine ?