On his Attestation Papers and in one of the write ups, it does state that he joined the 166th Bn. This is where it can get confusing.
The 166th was a battalion in the Queen's Own Rifles, enlistment in the Toronto and surrounding areas. Other battalions with the Queen's Own included the 83rd and the 255th. These battalions as well as several other were absorbed into the 12 Reserve Battalion on Jan 8, 1917. I went to the Matrix Project and checked to see what I could find. The 12 Reserve Battalion reinforced the 3rd, 15th, 20th and 75th battalions in France. These fought at Ypres, Mount Sorrel, Somme, Arras, Hill 70, Hindenburg Line and the Pursuit to Mons. Very impressive. Then I went to the nominal rolls to see if I could find which battalion he went overseas with. He was with the 83rd battalion which left Halifax, NS on the SS Olympic on April 28, 1916.
He is listed as
669275, Joseph Addison McNeil, NOK James McNeil, Elmsdale, Prince Co., PEI
I checked the nominal roll of the 166th and no McNeil's listed. The 166th was divided into two groups that left Halifax on Oct. 12 and 17, 1916 on the Olympic and Cameronia.
I haven't been able to definitively find this family on the 1881, 1891, 1901 or 1911 census although there are some good possibilities. (McNeill, NcNeil, MacNeil)
If you order his military file, you may find alot of information that you need. It will more than likely have pay allotments to either his father or mother which could narrow down that info. It will have complete medical documentation as to his injuries, when and where injured, what field hospitals and where he recuperated back in England. The records are very thorough. It will also have a record of all transfers from one battalion to another. Using that information, you can go to the War Diaries and probably determine what battle he was injured in. It will also give his arrival back in Canada and where his destination was. In some cases, it also has death info. I have purchased 3 files and have been extremely impressed with the contents.
There is a genealogy group in Elmsdale, PEI that may be able to help you find more information.