I think it's because his wife died..
So maybe before they could reproduce more the wife didn't made it and died..
I'm only guessing though.
If Akira did have more children it would only be because him and Shogo both want more. Right now he wants to focus on both Shouta and work so another child will have to wait.
I thought so when I read 1st chapter, but in the 2nd chapter when a mysterious middle-aged (The white-haired one) man approached Akira I was like "noooo, this man can't be Sougo's dad, noo... unless..." (=・ω・=)
I hope they don’t try and turn Akira into baby breeding machine also why didn’t the father just have more kids if he wanted the dominant gene to be spread it seems kinda dumb for him to hope that his son would just breed everyone and have a dozen kids