Hi E.,
I am a Lactation Consultant and have dealt with this issue many times not only with the moms I work with but also a little boy I have been sitting for once a week for the past year. He flat out refused his bottles for a few months taking no more than maybe 2-5 ounces per day IF we were lucky! He waited for his mom, slept most of the day and then nursed like crazy when she got home and throughout the night.
First don't worry! I know it is hard but really try not to. Your baby simply prefers you over any artificial nipple. Smart kid! :) Many babies will refuse every bottle nipple that is tried and simply do not eat (much) during the day - but when mom gets home they nurse nearly non-stop through the night to get their needed intake. This is called reverse-cycle-nursing. It can be rather disconcerting to begin with but moms get used to it fairly quickly. Simply sleep with the baby so he has ample access to you through out the night and he will be fine. :) This gives him the time to reconnect with you and get his meals. :)
There is one bottle on the market now that I use frequently with the babies I work with who need supplementary bottles. It is called the Breast Flow and is made by First Years. It is designed to make the baby 'suck' from it like they would from mom. They use similar tongue motion, jaw motion - the nipple is soft and squishy kinda like the breast. I have found it's statement that it reduces the risk of nipple confusion to be true. And for babies who are in your son's position, they take this bottle after a bit much easier than any of the others (that require an entirely different set of sucking skills than the breast, are very firm and stiff compared to mom's breast and are known to cause nipple confusion). For babies who are exclusively breastfed, trying to get them to take these other firm bottle nipples is kinda like giving someone who has only used silverware to eat a pair of chopsticks and expecting them to know what to do with them right off the bat - it ain't gonna happen! That poor person will be confused as hell at first. Some people will pick up how to use them quickly (baby takes bottle quickly with no problems) and some will never figure it out (baby who flat out refuses). :)
If you have other questions feel free to e-mail me at ____@____.com . Otherwise, I would try not to worry...he'll be fine. :) Just remember to keep pumping at work (regardless of what baby does when you are gone) to protect that milk supply!
Warmly,
J. Simpson, IBCLC, CIIM
www.breastfeedingnetwork.net