John Holmes is listed as a UEL in William Reid's book-The Loyalists of Ontario. His son Peter & daughter Deborah received land March 17,1812 through an Order in Council [Kingston]. These petitions are available on line from Archives Canada-just google Upper Canada Land Petitions. They will also be entered into Upper Canada Land Books-the book & page# given on the Petition.
There is also a UCLP for John Holmes-1808-Kingston, with some details on land granted to him.
There are also 4 pages in The Land Boards-Mecklenburgh-1790 for a John Holmes. This is digitized on line and you probably have to order them from a library to view them. I think this would contain information concerning his petition to become a UEL.
I checked the UEL site and they do not list a John Holmes as a Loyalist, although his children's Petitions state that he is. You could contact UEL Association for further information.
Hope this is of some assistance to you. Sandy