Many males have a very elastic penis that is much smaller when not at all erect than when erect or partially erect. I assume this was an adaptation for humans who originally lived in colder climates (to minimize heat loss). Assuming your stepson is old enough to at least understand the difference between an erect penis and one that isn’t - He should simply be told the above and that he does not turn you on.

I'm assuming you were simply flaccid? What was his mom's response though? I'm curious.

A little child who hears that it's going to get bigger can think it's going to become REALLY big, so anything the 'normal' size will still look small to him. I don't think a child is the most reliable source of information on what a good penis size is.

Joey, have you ever been in a Turkish prison...

And what is the reason? You can tell us!

both of my sons at this point have pulled the same stunt and seen me naked as well. it's normal for kids to question things. it can be as simple as "why do you have hair on your chest and I don't" but ultimately you need to explain to him that things change as you get older, and it's size doesn't stay the same either. depending on his age and maturity level you don't need to get into reason's why