I don't think my step son is my fiances child: Should we do a paternity test?

Should I get a paternity test for my stepson and fiance because they carry no similar traits and his ex went from man to man looking for a free ride? My Fiances son is 12. We are considering vasectomy because we both have one child from previous relationships and are focused on financial savings at the moment. However, my stepson looks NOTHING like his dad or cousins! They all have the same genetic traits except my stepson. Even my son carries so many traits of both his father and me I. Literally, my finance family, all their kids, look like copies of each other. All five siblings, all 16 kids. Yet his son has none of these traits. Even my family and all five daughters look very similar to our dad. His ex was a notorious cheater. Not just me being crazy, this woman Literally cheated over 50 times, and she laughs when she admits it. (She says in the text, “I’m such a nasty girl, that’s why I’m sick again,” talking to my fiance, referring to her current chlamydia and why she can’t care for the child). So naturally, I am extremely curious if this is his son. She doesn’t want the boy anymore, just the child support $, so we are getting full custody. She literally said, " I don’t want him; I’m too busy with my new kids. " She is living with her ex and his kids. They ALL look like clones of my stepson. (She dated this man when she got pregnant with my finances son). Same nose, eyebrows, cheekbones, and build. All of which look nothing like my financial family or all the nieces and nephews. My son looks more like my fiance than his son. If I could post a picture, I would! My stepson is a copy of these other kids, but nothing like fiance’s nephews or nieces given the distinct genetic traits. My fiance asked for a paternity test back in the day, and his ex freaked out. So, given her bs, my fiance didn’t get a paternity test done. However, before we get fixed, I want to know if he has a child that is actually his. If his son isn’t, yes, we will keep him. We love him and are his only safe place, but I want him to have a child that is his. He wants a child that is actually his and mentions it a lot. If my stepson isn’t my finances, he won’t get the vasectomy. But he also doesn’t want a paternity test. I was thinking about doing a 23 and me genetic test since it will show relatives and parents but also be informative on ancestry. (A great history lesson over the summer)

Poor boy. At this point, I would just get custody of him and let it be. That is his dad either way