Good to see that your baby has almost tripled his birth weight
. This indicates he is growing well.
Yes, as your doctor says, many kids on formula, pass green stools. You need not worry about that unless its watery and very frequent.
Babies drink to their satisfaction and they will not follow the quantity given on tins.
But breast milk is the BEST for your baby. The more you feed him, the more you will secrete. And expressing will never be as good as normal sucking. If you are still secreting breastmilk, you may try to restart feeding him. Take care of your diet- lot of fruits, pulses, dry fruits, milk, egg etc have to be taken. Take adequate rest. You will surely be able to feed him.
If you decide to continue with formula milk only, feed him with a spoon rather than a bottle- which can cause problems if not sterilized well.
Visit your Pediatrician
to assess his mile stones regularly.
Follow the immunization
Hope my answer helps you.