All the information given re registration is from Ancestry.ca
The original will be in the Ontario Archives. They will not do research.
If you know his death date you can get a copy from the Ontario Vital Stats at a cost. It should give his mother [if the informant knew the name].
I checked the 1861 census for McGillvray single women but there are too many without having a first name to narrow it down.
Witness on his marriage was a Miss McGillvray so that could have been his mother. I couldn't find him on 1891 census with a single older woman.
Did find this in 1891 census in Roxborough Stormont
A Donald McGillvray, cousin, aged 26, b Ontario, parents b Ont, labourer is with William McGillvray,23.
Here is the whole census item.
William, 23, Ont, father b England, mother Ontario, laborer
Donald, cousin, 26
Mary 44, father b Ontario, mother Scotland, servant,
Christy, 53, widow, mother, father b Ont, mother Scotland
Murdoch, 41, lodger, f Ont, m Scotland
Alexander, 27, lodger, f England, m Ont
Roderick, 16, lodger, Ont, Ont
William, 18, Lodger, Ont, Ont
Bella, 16, lodger, Ont, Ont
John,7, lodger, Ont, Ont
All born Ontario.
Daniel/Donald's birth won't be registered as registration didn't begin until 1869. Church records would be your only source.