I am assuming he is breastfeeding? Very common in babies. They just fall in love with your breasts and don't want anything else! It's very hard to break the habit -- but don't worry, your 7 week old will not hold out and not eat for as long as a 2 year old can if they don't want something. Most babies will refuse 1-2 feeding sessions in a bottle and then finally take it out of hunger. They have amazing reserve, so they really won't be "starving" or suffering. The longest I heard a breastfeeding baby go without a bottle feed is 9 hours. So my advice is to wait it out; one day, declare that you will not give baby breast for, say, 6 hours, only your milk in a bottle. It helps for you to be away for the day as well, so your baby does not not even see the breast and be constantly reminded of what he can't have for now. Often, the type of bottle/nipple you use does not matter; it's any bottle they don't like. After they get accustomed to the bottle they often won't refuse like that again.
In the future, I recommend that if a mom has to go back to work and is planning on bottle feeding, she should begin to introduce 1-2 breast milk bottles a day to get baby used to the idea, preferably at the times when you know your baby will be given the bottle (like during your work hours). This works for you too, since you can "train" your breasts to make milk efficiently via pump and it wil be easier to make a seamless transition to work while avoiding formula as much as possible.