Certainly a point of worry for parent like you when son is using Condom .Condom for masterbation may be used and may give some pleasure but this is not commonly heard or seen. Cetainly something fishy happning to him.Your child needs counselling
by parents and by experts for not to indulge in sex at this age when career is to be made ahead. He has ample time to enjoy sex legally in life. Checkout his activity,friends circle etc.Take him in confidence and warn him of consequences.Thesedays moble and ,internet pornogrphy is spoiling kids,even girls are becoming bolder .You have to be alert and act faster.