If your John died in Scotland, his death would have to have been registered and would therefore be found on ScotlandsPeople. The fact that he does not come up in a search, could mean that he has been recorded/indexed under a variant (or simply wrong) spelling. A search on SP for deaths of a John Scollan (with soundex on the surname) brings up three pages of possible entries, but reducing it to exact spelling 1891 to 1960 brings up four (with only two of these having John as a first name - the ages are out and one of them also has a middle name, which your John didn't - at least didn't at birth - he could have adopted one in later life).
(I just did a more refined search and found a John -Scallon- who died in the Registration District of Hamilton in 1902 aged 29, which would fit ... Have you looked at that? Or the John Scullion, 67, in Shettleston in 1939 - less promising but not impossible)
He could have emigrated (though a search of passenger lists on FindMyPast 1891 to 1960 doesn't show any with the right age. He could have gone south, but a search for him in the English census returns (1901 and 1911) doesn't show him there. He could, of course, also have joined up and been serving abroad.
I presume we are talking here about John Scollan, twin brother of Daniel, b. 2 Sep 1873 in Paisley to Thomas Scollan and Susan Duffy, who had married 3 Feb 1863 in Paisley? (And had issue - all born Paisley): Sarah b. 18 July 1863, James b. 25 March 1865, Thomas b. 27 June 1867, Peter b. 6 Aug 1869 and Susan b. 13 July 1871.