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The behaviour you are seeing is totally normal. It is how boars interact with each other and how they reinforce their relationship.
The biting of his bottom should not break Brownie’s skin and is called nipping. It is a gesture of power that Piggy is making to reinforce his dominance. It’s normal.
Please do not keep separating them. The baby needs to be with the adult for constant companionship as he is too young to be alone. Separating them is stressful to them and if separated too many times, it can actually harm their bond.
Separating them also means they need to reinforce dominance when you put them back together so you can actually cause an increase in dominance by separating them.
You only separate them is their bond completely fails and they have a full on fight (which it is not what is currently happening).
Otherwise for all dominance behaviours, you leave them to do it as it is entirely normal.
A Comprehensive Guide to Guinea Pig Boars
Dominance Behaviours In Guinea Pigs
How long have they been together?
Did you introduce them properly on neutral territory or did you just put brownie into piggy’s cage?
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
As you do have two boys, you need to make sure their cage is big enough. Boars are more territorial and need more space.
That is a cage measuring 150x60cm as a minimum but 180x60cm is recommended size.
Also make sure you have plenty of resources in the cage - two hay piles, two bottles (more than that is fine), at least two hides with each hide having two doors so there is always a way out if one piggy decides to block the other exit.
Cage Size Guide
Please also review their diet.
Their diet needs to be unlimited hay.
They can have one cup of veg a day (safe daily veggies are lettuce, cucumber, bell pepper and coriander/cilantro). Please do not feed carrots often. One very tiny piece can be given as a very occasional treat (no more than once a week) but carrot is too high in sugar to be in their diet regularly.
They can then have just one tablespoon of pellets per pig per day. Ensure they do not have constant access to pellets as they are the least important part of the diet and should not be consumed in large quantities.
Make sure you have several bowls or bottles around the cage so he can drink freely in a variety of places around the cage. I always ensure there are a couple of bottles right at the area where they hay is most.
You can also put bottles right by hides or under a covered area in the cage so he doesn’t feel exposed when drinking.
Making sure he had s good variety of vegetables in his diet will also mean he gets fluid from the veg. Lettuce and cucumber are watery veg so help provide additional fluid
Long Term Balanced General And Special Needs Guinea Pig Diets
I have added the green links to our forum guides throughout this reply. Do give them a read as they contain more information.