You are in the same boat as I was when researching the John Boyde who appears in the 1881 census in the Battalion in Salford. The only place to visit for this information is the PRO in Kew. The WW1 records were damaged by enemy action in WW2 (now known as the burnt records) and there is an extensive programme underway to film what remains. If you are lucky, the Es will have been filmed and your man's attestation record will be there. If you are unlucky, there will be nothing. As you have his service number, try checking the medal rolls too. If you cannot visit Kew - and you really need a day or two to acquaint yourself with the place - then there are plenty of military experts in the genealogical press who will work for you.