Erection at a weird time?

Hello all, I have a weird question to ask. At bedtime tonight, as my long time boyfriend and I were getting my toddler ready for bed, my son was doing this thing he finds funny which involves him sitting straddle style across our bellies and like bouncing kinda violently to make us like "shout" due to him bouncing on our organs lol. I'm sure if any of you have toddlers you'll be able to relate. However, tonight, my boyfriend seemingly got an erection from this. I really want to assume it's not from my actual child bouncing on him but instead an involuntary response to being bounced on in that way. I'm not stupid so please don't assume that and I'm not a troll either. This is the first time something off like this has happened, and other than this, he has essentially stepped up as a father figure for my child more so than his own father ever has. Should I be worried?