Please help! My 4 month old son barely eating anymore!!!

I have a general question about eating habits for your baby.

Have any of you encountered a time when your infant son/daughter didn't want to take their bottle?  I ask this because my son is 4 months old, and since getting his 4 month vaccination on May 4th, his eating habits have been thrown out of whack.  Prior to his shots, he was drinking 5-6 ounces every 3-4 hours.  Now he only drinks one 6 ounce bottle (during the night), and for the remainder of the day will have 1-2 ounces every 4+ hours!  Before anyone accuses me of neglecting my son, please be advised that this is NOT the case, as I want to have a healthy and chubby baby.  And yes, I have called my doctor about it and he insists that this is normal... 

I am concerned, because all he does is sleep for 2-3 hours after drinking about one ounce 🤔

Any advice would be greatly appreciated.

Thanks