Don't let this stress you out too much. The good news is that he is 6mo and food and dairy-based foods such as yogurts will now begin to supplement his milk intake.
I went back to work when my bf DD was 6mo. I spent a month beforehand trying everything to try to get her to take a bottle. She wouldn't, screamed the place down. Our last month together before work was a pitched battle. I was, frankly, delighted to be going back to work!
On her first day at nursery, I handed her over feeling wretched that she wouldn't take a bottle. She was fine. She would have a few sips from a bottle, and her mashed up food, and things like yogurts, but basically waited for me to collect her and had a huge bf then.
This continued for about six weeks then, bingo, she cracked. The nursery were very patient with her, they tried twice a day to give her a bottle, she would have an ounce or two at most, until she finally took the whole thing one day, out of the blue. It was like she finally realised there was no choice. But she didn't starve in the meantime and her growth remained fine.
How about introducing formula mixed with breastmilk, increasing the proportion of formula as you go along?
Talk to whoever will be doing your Childcare, explain the problem and see what they suggest. Hopefully they will tell you not to worry and will be happy to help you solve it.
I would say to try another formula, but personally I prefer Aptamil. I also think the closer to nature bottles are great.
We then had a huge battle to get her off the bottle when the time came!
FWIW, with DC2 we gave a bottle once a day from the first week and it has made the transition to childcare so much easier. You live and learn!