A James Merrill of Malahide [township in Elgin County], made a Petition 1818 for 100 acres of land. It gives his father's name as William. There is also Petition made in 1818 by a William Merrill of Yarmouth to lease 50 acres of land. It states that William is about 60 yrs. old, fought in the last American war for the British and has been in the province for 12 years, having immigrated from New Brunswick. It's a possibility he could be Jame's father as Malahide and Yarmouth are both located in southwestern Ontario-the main city being London.
You can access these petitions on-line. Google- search Upper Canada Land Petitions 1763-1865.
Just type in surname only.There are 17 petitions made by Merrills. The Colonel Thomas Talbot referred to in the petitions was the Land Grant Administrator for southwestern Ontario.
The rebellion you referred to was the Upper Canada Rebellion of 1837-38. Rebels were treated harshly. Some were hung or or shipped off to Australia and many escaped to the States. A general amnesty was granted in 1845.