Feeding for baby. This has started!

My baby is 3 weeks old in the first few days he had. A problems with latching then I used to express milk and feed but slowly he can suck now he is sucking but now he is 3 weeks old going to 4 weeks. Previously after feed he used to sleep atleast 3-4 hrs and then get up and now he wakes up after feeding and cries, am feeding him on demand everytime but still whenever he leaves the nipple he sucks his finger, and then again I feed him, also it needs time for making milk, so after breast feeding am giving a bit of formula milk then he remains quite again wakes up after 2 hrs. May be he is growing this is the reason but am trying to keep him just on breast feeding atleast for six months, so bit of confused what happened. Pl suggest. Am feeding on demand my milk comes out but don't know how much, I don't want to take any medicine for milk increase want to do it naturally. Even am not sure whether I need to increase milk production. So don't know of late since yesterday this has started