Re: So how do I covince him?
Tell him that the amount of water spent washing diapers is equivalent to the amount of a child using the bathroom each day. Does he complain that an older child regularly using the bathroom is a 'waste of water'? Then he shouldn't worry about diaper costs water-wise.
FWIW, Our water bill has risen by about $2.50 per month for washing our diapers, and our electric bill hasn't really changed at all that i've noticed.
DS - 6 & DD 3.5