The type of program that will meet your sons needs sounds like an overnight summer camp. Camps.ca has more than 200 summer camps in Canada. You can choose an ESL (english as a second language) program if you want to focus on language studies, however one of the strong benefits of summer camp is that kids learn english without even realizing it because they are having so much fun. Try Frontier Trails Outdoor Education Centre.
For a complete list of results, use the advanced summer camp search, and select the criteria: 6 years old, overnight, horseback riding and ESL. You'll receive a complete list of results that match your criteria.
Here are some good horse back riding camps Here are some good ESL Camps