First, you should check English school board of Montreal to see if your kid can study in English public school here. Don't simply based on what you heard from others but directly ask the board yourself.
You should also check some private English school like LCC for your kid if you can afford the tuition.
Now French school, anyway you missed the chance to put your kid in secondary 1st in September this year, there is no chance for him to go to any secondary school which need a entry exam this year. I don't know if it's an option for your son to choose welcome class or not due to his English background. You have to ask Montreal French school board to make things clear. Keep in mind, if your son under 13, he can stay one more year in six grade, either welcome class or normal class. If he is over 13, he will have to enroll a secondary school; they should also provide welcome class. No matter what, talk with school; don't put him in welcome class if possible. Normal class(even in the same grade of welcome class) for him should be much better than welcome class if he alread know little bit French plus a good English. Good luck!