Hi, welcome to the ABW Forum. The 10th Northumberland Fusiliers to my knowledge was a WWI NF battalion formed in Sept 1914 in Newcastle upon Tyne as part of Kitchener's army. I stand under correction but I'm only aware of the 1st & 2nd Battalions of the NF serving in the ABW.
I can find a WWI medal index card for 9358 Cpl. Stephen Sanders of the Northumberland Fusiliers. He seems to have later joined the Royal Engineers with numbers WR/276572 & 308444. The MIC shows his medals as 1915 Star, British War Medal & Victory Medal.
There are several men by his name in the QSA rolls for the Boer War. Without knowing his regiment one can't identify which man is your grandfather. If you are able to give more details on his Boer War service we may be able to find something on him.
Have you searched Find my Past or Ancestry for possible service records?
Cheers,
Mark