Switching to formula

So after 3 months of exclusively breastfeeding and 2 months of exclusive pumping I think I'm finally ready to switch to formula and I think I'm finally at peace with that decision. My question is though will 5 month old baby suddenly put on a lot of weight? He's always been on the smaller side. Also what if I still pump twice a day? Will supply dry up? What formula do you think is best? He's had 1 bottle of similac advance every day but I'm worried it will constipate him