There is a Donald Mcculloch from Oban living in Glasgow on the 1901 census. Two different people on two different returns.
The Donald Mckay(Mcculloch) one dosent seem to be him. If he had a birth cert it would more likely be under the name Mckay(Mackay). It looks obvious that they were born Mckay. On scotlands people there are 2 hits for Donald Mckay in 1884, and 2 in 1885. and one for Eric in 1888 in Durness.
Another thing to do is to search out the 1911 census, see what Margaret(Maggie) is doing, maybe she moved in with Thomas Mcculloch.
There is a Canadian WW1 sign up for a Donald Mackay, from Durness birth date(2 Feb 1885) The problem with the Mckay name is its like Jones and Smith in England, very common name. which could lead to confusion,
step 1 is to get the correct birth cert, then compare dates, see if it is the same fella.
mm