How to break the news

Im a (40yr old)mother of 2 boys and, been separated from their father for many years but divorced was finalized a couple years ago. I havent dated anyone or brought any male into my life until recently. I started a relationship with a (49 yr old) man who I have known for many years. His son is best friends with my oldest son. My son has went on many vacations and outtings with him and his son over the years and my son has always liked and respected this man. We have kept it casual for 6 months (none of the boys know this) and now we officially want to start dating and make this into a relationship. Beings our 2 boys are basically adults, living their own life, and they both work off and never around it's hard to get them all together and talk to them about us. This man is also worried my oldest will change his feelings toward him, he loves him like his own and don't want him to end up hating him. And also they are not 10 anymore so it shouldnt be a big deal. How do we approach this? Do I tell my 2 and he tells his son? If they don't like the idea, how do we handle that?