Not happy with pants size 5

Hi I am not happy with Huggies pants size 5. They are small on my son. I used to buy nappies size 5 and I wanted to try the pants and see how are but I'm very disappointed because it's a size 5 but too tight on my son. Now he has heat rash. He has big thighs but it's a size 5 it's supposed to fit him as well as the the nappies size 5. I wanted to send an email in the Huggies website but the subject line is not taking my subject of the message therefore I can't send them a direct email. Their Facebook page limits you when you write a message. I am really not happy with Huggies. How will my son finish the pants if they hurt him.