Hello K.-
Don't stress. I know it feels terrible now, but it will all work out! I went back to work after 3 1/2 months. My son was doing ok on the bottle (with expressed breast milk) until about one month before I had to go back to work. He just refused to drink from the bottle. I think he must have felt my anxiety and knew something was going to change.
You might try an Adiri bottle. To the baby they feel like a real breast. Some people swear by them. They are expensive, but if it works any price is worth it!
When I did return to work, the first week with his grandparents was rough, especially the first day. It is difficult, but your son will get into the routine and he will be perfectly fine. Just know that when you get home from work he will probably want to nurse, almost, continuously. I know my son did. But it is a great opportunity to get reconnected after being away during the day.
I have been pumping at work now for three months and it is going smoothly. I pump 2-3 times per day. I am at work 30 hours per week M-Th. I have not had any problems with low milk supply.
My husband and I are now moving on to the next dilema. Our son's poor sleeping habits. But, I'm sure this too is just temporary!
Wishing you well,
M. Z