Formula for 2yo

My boy is 28 months now. He has been drinking Nan Optipro since birth and a month ago we changed to Pediasure Gold. Ever since he started on Pediasure, we noticed that he’s always constipated. Hard stool. He would tell us “Poop pain”. So we’re planning to change to another brand. We’ve tried giving him fresh milk but he didn’t like it and refuse to drink. He only wanted the flavoured ones. Any suggestions on other formula brands? Thank you!