I had the same challenge -- my daughter is now 7 months old -- and worked with the folks at the occupaional therapy department at Children's Hospital to work through it.
In short, what they told me was three things:
1) the goal while you are working is to get enough milk in him so that he doesn't dehydrate. your doc or a lactation consultant if you're nursing can tell you what to aim for. my daughter was 20 weeks when i went back to work and our goal was 6 ounces in the 8 hours i was at work -- so not a lot, just to give you perspective. don't stress is he is isn't taking what he would normally take from you during that time.
2) that said, expect him to reverse-cycle when you are back together -- in other words, nurse more once non-bottle food is available. for me, it meant that my daughter who was on a every three hours schedule for nursing before i went back went to nursing every 2 hours for a while. she was just making up the amount she needed to given how little she was eating during the day.
as the OT explained it to me -- think of your son "sleeping through the night but during the day" -- he can go longer stretches, it's just that he's going to do that while you're at work.
the downside is that you will get less sleep.
3 the last thing they told me -- and this ultimately worked -- was to work with different delivery mechanisms - not just a botte. P would drink from a spoon or a small (shot glass, really) cup in very very small amounts. when she got older we took her to a sippy cup at about 6 months.
lastly, the daycare people helped immensely -- the same two people gave her a bottle every day and were incredibly patient. now, at almost 8 months, she will take a bottle from those two (we're up to about 14 oz a day now -- yay!) but still no one else, including me or my husband.
so -- three pieces of advice:
1) aim for minimal amt of fluid during the day
2) allow him to reverse cycle
3) try different delivery mechanisms but keep the people consistent, if you can.
GOOD LUCK -- I know it's stressful on you. I cried and cried and cried and worried, but the options that the OT gave me helped us find a solution that worked for us.
And if all else fails, ask your doctor to give you a recommendation to Children's Hospital for an evaluation if your insurance will cover it.
Keeping my fingers crossed for you.
L.