My dog is pee pad trained when I try to box train him, by putting the pee pad in the box, he won't use it, why and what can I do?

Read through the sticky at https://www.askmehelpdesk.com/dogs/information-articles-our-dogs-expert-labman-53153.html#post251809
It is mostly about outside, but pick out the parts on knowing when to take the puppy outside. Watch for the signs he needs to go and take him to the box. Use a command. Be ready with the praise.