Dear Ms. Bird,
Thank you for posting your request on History Hub!
In researching your Grandfather's service, you may wish to reach out to either of the two agencies listed below.
For the Seaforth Highlander Regiment of Scotland
The Gov.uk website has information on how to request a service record here:
Also The National Archives of the UK has information on how to research service members from WW1 online here:
We hope this is helpful.