Touchy subject TMI

So this is a very touchy subject and I really need some advice or maybe possible outcomes or something.. My SO and I have been together for 5 years. We are 5 months pregnant right now and his grandmother and I were discussing the baby shower.. she mentioned to me a family secret that my SO's mother kept her entire life.. apparently the man that had taken care of my SO his entire life isn't his father.. instead.. His uncle is his father... I know it is absolutely disgusting to think about but his mother and her brother created the love of my life... it's f-ed up.. My SO has no idea his father isn't his father.. he has no genetic problems that we know of and he's completely normal so I would have never known that he was made by incest.. i guess I want to know how to tell him and if our baby will have issues.. I'm sorry this is so awkward..