My aunt has that gene, & my baby cousin came out with 12 fingers, which the extra fingers were really small next to the pinky ..but they got rid of it once she was born & it doesn't even look like she ever had it.
My husband had twelve toes too. Its a dominant gene unfortunately so I worry about it too.
My husband has a guy working for him who has three fingers on one hand and four on the other they just delivered a baby about a month ago and the baby had ten fingers and toes the guy was worried about the same thing but like he says his baby came out perfect
It is based of genetics and its just the roll of the dice honestly. Do your bf parents have it? If so theb it could be dominant but id not your genes might play the probability smaller.
Polydactyly is genetic and I do believe it is considered the dominant trait so it is a possibility.