15802 Frank Willis could not have been with the 7th Battalion for all of his war, as he went overseas in February 1915, long before the 7th battalion originally went to France (1-9-1915). He would therefore have previously served with the 1st or 2n battalion, and probably was invalided home with wounds or sickness, before returning to France to join the 7th Battalion.
He is not listed in the original Nominal Roll of 7th battalion men going to France from August 1915, so his joining the 7th battalion would have been after September 1915.
He would have enlisted into the Army in September 1914.
Attachment below is from the 1914-15 Star Roll of the Northamptonshire Regiment, Page B362. I don't have his details from the Victory/British War Medal Roll.
I hope this helps,
Steve.