Tell him the truth; they won't work. Truth is, there is no proven way to enlarge the penis. It's not uncommon at all for guys to worry about not being "big" enough.

The best thing I can think of would be to just keep telling him its the right size for you, and if you really want to boost his ego then the next time you are intimate with him really let him know how much you enjoy it ( if you get my drift ;) ), other then that if you tell him enough that you like it the way it is then it should sink in with him over time. Hope you find my advice useful and good luck!