Trying to find out if my great grand-father served in WW1, Directory of same?

Question:

Trying to find out if my great grand-father served in WW1, Directory of same?

Born in Crowborough England 1889, moved to America between 1905-1914. Left me with some artifacts that suggest he was in WWI. Name: Iden Tomas Southon. Also was part of the American Legion in the 1960's-New Jersey. Passed away in 1978.

Best Answer - Chosen by Asker: This looks like him:
Canadian Soldiers of World War I, 1914-1918
about Iden Southon
Name: Iden Southon
Residence: Madison, New Jersey, United States of America
Birth Date: 28 May 1891
Birth Location: Sussex, England
Relative: Lottie Southon
Relationship: Wife
Regiment Number: 2304445

The image is stamped at the top:
1st Depot Battalion
1st C.O.R.
Attestation Paper
109th Regt. CEF
Canadian Over-seas Expeditionary Force

English
2304445

On the form, he also declares previous military service:
Q: Have you ever served in any Naval or Military Force?:
A: Yes, 5th Sussex Regt. 4-1/2 yrs. pte.

Date: July 17, 1917