Are his droppings smelly? How do the droppings look like, are they perhaps softer than normal? Is he eating and drinking properly? To be sure he hasn't got an illness, you should collect the droppings 2 days and take them to a vet for an examination (e. g. coccidia or worms).
Two years ago I had got a new guinea pig whose droppings were smelly and it didn't eat properly. The examinations (three times, because the 2 first ones were without diagnosis) of the poop showed one (only one single !) worm egg. But the anti-worm-medication helped very good: two day after the medication started, the smell was away and my little piggy started to eat with a good appetite.
Sometimes it is very stressful for a guinea pig when it comes into a new home and then it can become ill. You should take the litte boy together with a collection of his poop to a vet to be examined. Guinea pig poop normally does not stink!
All the best for the little boy!
