I found this thread*I would this thread about impaction from admins, hope it helps! Poor piggie bum.
Impaction Recovery - How To Help Your Guinea Pig.
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Heres a better pic I'm sorry that it isnt the most pleasant thing to look at. It smells really bad- please help!
It's less common for younger boars to have impactions.
What do you feed him?
Does it happen regularly, or is this the first time?