hi i am bf my 4 week old son and in the afterno...

hi i am bf my 4 week old son and in the afternoon i dont seam to have enough milk, has been like this for about 2 weeks now and i think its getting less and less milk he is still putting on lots of weight, wetting and pooing lots just so upset of an evening its not funny. some people say give him a supplement feed of formula, but midwives say give nothing but breast. can anyone help me please