Certainly historically there have been drummer boys in the army
This is wrong I am afraid. These were two separate ranks. When a Private earned 1s a day, a Boy (under 18) earned 8d and a Drummer earned 1s 1d a day.
Before WW1 you enlisted in the army for 12 years. You could serve for 3 years with the rest in the Reserve. But the three years started at the age of 18, so a Reservist would have had to be 21 years old. And as a Reservist he would have needed permission to emigrate.
The numbers just do not stack up.
He may have joined the TF (Territorial Force) in 1910 at the age of 17. Served the minimum of one year, and lied to ‘Company A’. But then why would the Americans believe an 18 year old could have six years soldiering experience?
If he was discharged before WW1 his papers may be in Kew. If you cannot get to Kew Findmypast will be putting them online by 2011.
What age did he give at the time of his US marriage? And have you see a scan of the cutting, or has it been transcribed?
Ken