11 day old son stopped breast feeding
I would try to get baby on your breast, it is so early in the game that your supply should catch up nicely to meet all of baby's needs.....if that was a concern. Do you cosleep or baby wear? The reason I ask is that both offer many opportunities for baby to latch on without mom having to think much about it.
I am by no means an expert, but I would at very least do away with artificial nipples and keep offering him the breast. I would assume he would take it eventually.
With that being said, milk production works on supply and demand. Have you been pumping while he gets formula up until now? Can you?
If you still feel the need to use formula, perhaps consider cup or dropper-feeding. Put him to the breast anytime he fusses. It may be every 15 minutes and THAT"S OKAY. Spend lots of time skin to skin and get some local real life support.
Good Luck, mama!