What can I do for my 5 mth old baby who I think is constipated?

My baby is on formula milk. I was feeding him s26 but recently changed to heinz nurture because the nurse told me s26 can make them constipated. Since I changed formula milk my baby doesn't seem any better. He'd be lucky to poo every 3 days and he seems to be in pain straining all of the time. He seems to be ping wind a lot aswell and that seems to be bothering him. I've tried a bit of water but he doesn't seem to want to drink much and I've tried something called coloxyl drops that are supposed to soften the poo and relieve constipation but nothing seems to be working. What can I do?