Similac Alimentum

My son was on Similac advanced and he started having really bad mucus and stringy poop so the doctors changed him to Similac Alimentun thinking it's a milk allergy.....He has been on it a little over a week and is constantly constipated...been giving him karo in his formula to atleast get one poop a day..He also hates the formula he went from eating 3-4 Oz every 3-4 hours to I'm lucky if he eats 2oz every 4 hours he also seems for fussy and I have to give him gas drops like 4 times a day to help with the has its giving him..I don't know what to do and I'm scared to keep switching the formula please help with advice maybe some other formulas ro try...has anyone else had this problem???