Imagine this scenario:
Down the road you and your boyfriend decided to have children. There's a terrible car accident and your child needs a blood transfusion if they are going to live. Imagine he responds before you do (perhaps you are unconscious as well). Will he allow the child to have a blood transfusion?
Heck...imagine YOU need the blood transfusion to save your life and are unconscious. Will he try to prevent the blood transfusion?
What about fractions of blood? Will his conscience allow them? What if the fraction isn't a "conscience matter" as defined by Watchtower?
Imagine this...what if he takes the kid the Kingdom Hall on occasion? Or decides he wants to have someone at the Kingdom Hall "study" with the kid (perhaps secretly...this has happened). Imagine this...the elders are aware that one of the JWs has molested children in the past, but have not warned the parents. In fact, that JW still goes door-to-door. Or perhaps the elders have heard of reports about a JW, but because there aren't "two witnesses" to the molestation, their hands are tied and they can't do anything about it.
There are all sorts of nightmare scenarios if he is not mentally out. His parents would be actively attempting to study with and makes sure any children become JWs.