could the new addition have caused stress perhaps? although you say they get on well so this sounds unlikely. the new piggy could have brought an illness with them which could have affected him aswell although most of the time, the quarantine period will be enough to stop this from happening.
is he eating at all? and is he pooping? it's important to make sure something is coming through if you will be syringe feeding. if he gets any worse before tomorrow then take him in sooner. their digestive systems need to be constantly moving and if he has already done 5 days of reduced eating then this is worrying
keep a very close eye on him tonight and if you feel he is getting worse, don't hesitate to take him in sooner. the sooner any condition is treated the better. good luck.