I guess you could train Khan to use a private area for his poops, might help if you put a bit of one of his poops in the area to encourage him to to there, along with a square of turf that he usually chooses to wee on. You'd probably have to take him to that area each time on a leash, and wait for him to perform, using a cue word such as "Hurry Up" when he goes, and click and treat every time he performs where you want him to go.
I did once try to train one of my dogs to use a particular area, but never had much success.
My dogs have definite areas they prefer to use, Keith goes by the dustbins and at the gate to the field, Stav usually goes just inside the main gates, to mark territory.
Albert, like Keith when he was a puppy, goes by the flowerpots, as far 'inside' the territory as he can get.
So, if you notice a specific area that Khan prefers, you could put his Loo there and hope for the best.
It might be easier if you simply 'fenced off' an area of your garden and "dedicated" it for the guinea pigs use, making it 'out of bounds' from the dogs' attentions, that way you'd save yourself a lot of hassle
