B.H.
To prevent them from being stinky you'll have to wash/dry them often or dry them everytime he comes in the house.
To get rid of the stink right now try this fabric softener it's popular with the latino crowd. Suavitel is the brand name and it's super potent smelling seriously. You might want to try the other's ideas of soaking in hot water and then try this fabric softener I swear by it.