Pumping and storing

Steph.
So my DH doesn't get why I need to store milk. If when I go back to work I pump while there then that should be enough for the next day  
I think this week my LO isn't staying satisfied as long. Does that mean my supply is decreasing?  I let him go until he stops. Normally anywhere between 10-20 minutes. When k pump after I get less then a ounce from that side.