I think, if he's still hungry, then give him the odd bottle, or even one once a day. I don't think it would affect your breastfeeding if the bottle is extra to what you already do. Have you tried expressing a bit after each feed? Perhaps a little on each side and collect it up and give him that as an extra feed each night? Hopefully there are lots of people to help you darl, its not your fault bubby is underweight. You can only do what you have to do. Good luck sweetheart!