When I was still in my process of getting a US visa, I've encountered a lot of people who are also undergoing undergoing the same process as mine. There's one certain person who also had a son like me, but his son is his US citizen fiance's son. I remembered her specifically coz she was worried about her son may needing to undergo DNA testing to prove that he's a legitimate son of her fiance. But I don't think they did it. Well, if I were to say, the child looks so much her fiance so there shouldn't a doubt that it's his.
In my case, my son's not my husband's legitimate child so I was never worried about it. Also, his biological was out of the picture coz I never had his name on my son's birth certificate.
If you went to a DNA testing process when you applied for your US visa, please let me know so that I would be able to help anyone who would come to me and ask me about this concern. Thanks and have a great day!