I have been breast feeding my baby full time un...

I have been breast feeding my baby full time until he was three months. Then I returned to work and have been expressing my milk. I would make up the difference with formula, if I didn't express enough. I work for 3 days, I feed him in the morning and at night. The other days he gets feed off me. When I express lately I only get 60-100mls per breast. According to the formula can he should have 240mls. For 2 Monday nights in a row he has woken during the night (he has slept through the night since 6 weeks), he had his last feed at 10pm and feeds for 1/2 an hour. I was wondering is my milk running out? as I don't know how much he drinks when he is breast feeding. He gets 3 small solid feeds and 4 milk/formula feeds per day.