Expressing is not a good measure of how much milk you have - just because you don't express much doesn't mean you don't have much milk (everyone is different when it comes to expressing, from what I've been told).

Better indicators of whether you have enough milk:
* Is he satisfied after you feed him?
* Does he have enough wet & dirty nappies?
* Is he gaining weight?

How old is your bub? Is it possible that you are just not feeling the let down? It took a month or two before I actually 'felt' the let-down. Do you notice a change in your bub's sucking (ie. changing from quick little sucks before let-down to slow deep sucks after let down??).