Baby wont take more than 3-4oz?

My son is 18 weeks and since birth he wont take more than 3-4oz per feed. He wakes up twice in the night for feeding becuase of not taking as much as he needs in the day. Its wearing me out as im on about 4-5 hours of sleep a night and I have a 4yr old daughter as well to look after. I just CANNOT get any more milk in him when he feeds during the day.