It is totally normal for milk not to come in till 3 days (if fact that is about the earliest it will come in) and he lost a normal amount of weight too so it sounds like he did get colostrum but has struggled since your milk came in.
1) before you latch him, if you can, get your breasts to let down in to a cup or cloth (by manual expression or pump but don't keep pumping, just to get letdown going). Then when you latch LO he won't get suddenly overcome with a high volume of milk.
2) block feed him (every feed within a three hour window should come from the same breast - any longer than that and you risk engorgement in the other breast)
3) read up on normal and problematic breastfeeding signs on the LLL website or Kellymom.
4) see if breastfeeding charities like LLL would come out to visit you and observe feeds (this was a lifesaver for me)
5) stop pumping - it is confusing your breasts in to making too much