He is your ex if he did something to hurt you and especially if he broke things off with you he does not deserve an explanation. My ex was the biggest jerk and recently called me saying he missed me and how much he needed me in his life and this was 2 months after we broke up. Not only that but I think it was because he saw pictures of me enjoying myself and being happy. I found someone who has made me happy and you know it's my ex's loss. Girl you deserve better and remember as long as you're happy that's all that matters :)
Don't lie and deceive your ex. Just be honest with him and tell him you are no longer interested in a relationship and not to contact you. If he asks why, just repeat your original statement. Hearing it enough times, he will get the message.
He's your ex, you don't really have to say anything to him. But if you must, just tell him the usual welcome home polite conversation but if he brings up you two again just tell him you've closed that chapter of your life and moved on and he should do the same, put him in friend zone, he'll get the picture.