My 3 yr 8 mnths boy had a -1 cylindrical power is tht necessary to wear glasses?

Not necessary. If the eye (s) is otherwise normal in all other respects an there is only a 1 diopter of cylinder, than eyeglasses are not necessary. You might be being victimized by an aggressive optometrist who just wants to sell eyeglasses. Children this age should always be seen only by a pediatric ophthalmologist.