Hi!
Excited boars will mount annother piggy from any angle irrespective of gender. What you should do is create a little refuge with two exits and a little soft hay inside that only the baby can get into; in a pinch a small tunnel cardboard box will do. Boars don't hurt babies but they can go overboard with the humping so you need to give the little one a chance to have some time out whenever needed.
The screaming is submission screaming, not pain. It is the correct response to dominance mounting. If the baby has had enough, he will either run away or challenge your boy.
Make sure that the boars have plenty of space and no dead corners in the bonding area as well as in the cade wait until the initial overexcitement has died down before transferring them to their cage. Remove any huts with only one exit but make sure that the boars have two of each (huts or tunnels, water bottles, bowls which you remove between meals and access to hay that cannot be dominance blocked). Keep in mind that it takes about two weeks after the introduction until the group has been fully established.
Please read the chapters on boar bonding and baby bonding in our bonding guide:
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
Here is a list of dominance behaviours:
Dominance Behaviours In Guinea Pigs
Our boar guide contains a chapter on the do's and dont's when it comes to boars in order to not upset them, especially during teenage. You should find that helpful as it removes the most common issues for trouble:
A Comprehensive Guide to Guinea Pig Boars