my husband met his biological father at the age of 21, i think it was by accident that he found out that the father who he was named after, was not really his father after all, and the bio father who had 7 more kids, didn't want anyone to know about him. his mom didn't bother to mention that his father was not his father until some little girl playing around in the yard told him that she knew who his dad was and he asked his mom was what the little girl said was true and she finally admitted the truth and then wanted him to meet his real father, who lived in the neighborhood all along.
don't know what the moral of the story is, but just wanted to share how my ah has resented his mother since then for not at least telling him the truth about his father. i don't think that this is the same thing though. i guess i think maybe if you did mention it, then it wouldn't be on you, i know you have to think about how your sons will feel about him calling, but how would they feel if they found out somehow that he did call and you didn't tell them.
i really think that it up to you to decide whats best for your boys, i think gentle would be the way to tell them, if you decide to tell them. i think maybe you could give it some deep thoughts, and you decided what is best for you to do.