My grandfather'd WW1 uniform mystery

[Cliffhanger]

Hello,

New bloke here. In light of tomorrow's momentous anniversary I'd dearly like to identify my grandfather's role in WW1. I have two photo's one of wigh shows collar/lapel badges (possibly a stag?) and also a lower sleeve stripe.

He name is Alec (Alexander) Jack and was born in Manchester in 1891 of Scottish parents.

Here's hoping someone can solve this long-standing mystery.

Many thanks,

Iain Jackimg755 (3).jpg