Thank you all for your help so far.
His grandsons still do think both pictures are of their grandfather. To me, in the picture with the lorry he looks around 18-20 years old which fits with someone in WW1 who was born in 1898.
In the picture of him in the Scottish uniform he looks to be 14-15 years old, a lot younger, possibly too young to be in the Army. Tonepad has already said the tassels aren't correct in that the number is unusual. He is also carrying a pace stick. Isn't this associated with someone older and of a higher rank?
So I am wondering if this is a red herring and it is a costume not a uniform. Carnival in Malta has been massive for hundreds of years and there is a lot of dressing up. Before we all had cameras and mobiles people used to go to studios to be photographed in their costumes and this looks like it could be a studio photograph. Wonder if this is a possibility? The Highlanders were in Malta after the Boer War so that would explain why the uniform was here.
I have got the boys searching for medals now so if they are still in the family I will be able to confirm the service number.