I have an idea where you go from here.
You tell him that you will be contacting the police and getting the computer checked so that you know what he has been accessing. Because you have no intention of now not doing that and putting yourself in a position where, if at a later date he is found to have been accessing child abuse images, you do not want to be explaining to social services that yes, you knew he was into 'teen' images, but did nothing to make sure your children were protected.
What you saw is not necessarily the full story of what he has been accessing.
The important point is, surely, that he is turned on by and actively seeks out images which focus on young girls. Whether that's 17-19 year olds or younger, you do not yet know. But you need to FIND OUT, not try and ignore this on the pretext that what you saw wasn't technically illegal.
I would be livid, but most of all devastated and worried. Because I cannot get away from the fact that perhaps of all the different 'groups' of men, those who have teenage daughters living in the house with them should be, surely, among the LEAST likely to find images of specifically YOUNGER girls/women a turn on. Precisely BECAUSE they have a teenager in their home, doing normal teenage things, demonstrating with every interaction that actually, they're still children. Just older children.
He is seeing these interactions and taking part in them every day and yet this is his choice of porn?
Please don't keep this within the 'safety' (for him) of your family unit.