HI E.,
First of all, I feel your pain about going back to work, I truly do! It was one of the hardest things I've ever had to do and it still isn't easy! The 2 days before I went back I was a mess - crying continuously. But, we have to do what we have to do and I just cherish the few hours I get with my daughter when I come home from work and on the weekends. Good luck with his first week at the sitters!!
My daughter had horrible gas since the time she was born which caused her a lot of discomfort and she was very fussy. After I stopped breastfeeding, which I thought was the culprit, I tried the enfamil gentleease formula too which didn't work. Finally, we just tried Soy and she was a different baby! She had very little gas and didn't spit up as much anymore. We use the Parent's choice brand from Wal-Mart which is about $13 and you can call once a month to get coupons. I think its actually made by Emfamil.
In regards to how much to feed your son - our doctor always said to feed babies whenever their hungry and as much as they will eat. When our daughter was taking 4 oz at a time at her 2 month checkup, he asked if she was finishing the bottles. He said if she was finishing the bottles, then increase to 5 oz and so on and so forth. I think its better to feed more in one sitting and have him eating further apart which will also help you at nighttime and getting him to sleep in longer stretches. My daughter now eats 8oz every 4-5 hours but sleeps 10-10.5 hours at night. We were getting 6 hour stretches out of her by 2 months old.