Unfortunately, SP do not allow for birth searches using known parents names (would that they did).
It's not so much that they don't
allow such searches, but that the indexes were compiled originally without mothers' maiden surnames until 1929. SP has been gradually adding mothers' surnames to birth index entries, but progress is very slow.
Hats off to the GRO for adding mothers' maiden surnames to their indexes, though they still have some way to go too. I found a lot of index entries yesterday without mothers' surnames.
(When the GRO has finished with adding mothers' maiden surnames to the indexes, wouldn't it be nice if they went on to index full middle names from 1911 onwards!)
Is there any easy way to find out whether this boy had siblings?
Not really.
As you presumably know his mother's maiden and married surnames, you could start by looking for her death (assuming that she died in Scotland). If she died quite young, that would give you an earliest and latest possible birth date for siblings (leaving aside the possibility of an older child born before the parents' marriage. Then search the district where the 3-year-old-in-1891 was born for births with the same surname.
If she died after 1911, go for the 1911 census. This will tell you how many children she had had, and how many were still living, so you know how many you need to find.
If any of them were living with her in 1911, you will know where they were born and thus where to look for births. You should also be able to find her in the 1901 and 1891 censuses.