No, if you are using an outside stud, he does not have to be registered with ESCR, although you will have to submit the outside stud’s pedigree and registration information for inclusion in the ESCR database. Submitting this information does not mean that the dog is registered with the ESCR, it simply means that the dog will be included in the pedigree database. You will need that information later to register your litter.
Please note that outside studs are required to conform to the same health test regulations that a stud owned by you would have to.