Pumping procedure

I formula feed my son who is 7 weeks old after breastfeed him five mins each side since my milk secreation is not sufficient for him. I heard after baby stops sucking if pumping is started it will increase milk supply. My son used to stop sucking and leave my breast immediately after the milk supply stops from breast. Then he used to cry a lot since his stomachs is empty and I used to feed him with formula. Can I start pumping after he finishes his formula feed which takes about 20 minutes after breast feed ? Will it still help to increase my breast milk quantity? Or should I pump immediately after he stops sucking from my breast?