A bit more on the Sidney G Young as mentioned.
Attested 10/12/15 into the A&SH as ? & later given no. 253225 which was his post 1916 no. as all TF Batts. were re-numbered early 1917.
At some point after this he was transferred to the 6/Seaforths no. 285204.
Discharged (sickness) 8/11/17 aged 32y & 272d.
Does this sound like it could be your man?