Are you sure it's from the close encounter with the mouse trap? Either way, if you saw something looking like pus I would get a vet to take a look. I doubt you want to put anything on that would put pressure on it as likely that would cause more pain and if there is an infection you want what ever is in there to come out not stay in. Relieving pressure relieves pain but you want to make sure it doesn't get in to his blood circulation.
If you want to try and keep the area clean until a vet can take a look try using a very loose gause covering over the paw and tape around the leg more firmly to make it harder for him to chew off - chances are he will try to remove anything you put on though.
If you want to try and keep the area clean until a vet can take a look try using a very loose gause covering over the paw and tape around the leg more firmly to make it harder for him to chew off - chances are he will try to remove anything you put on though.