Do some children gets an enlarged spleen with all viruses? My son already had mono but he seems to get a slightly enlarged spleen whenever he's sick.

Not with all viruses. Mononucleosis does cause splenic enlargement but all viruses do not cause splenic enlargement.But there are others beside the Ebstein Barr Virus which causes mononucleosis.Cytomegalo virus causes it.But you say it get enlarged whenever he is sick.I suggest you discuss with your pediatrician if it is of concern.If it goes down after his illness .then I see no problem.Discuss with his Pediatrician.