When I moved to Canada, my kids did not speak 1 word English. They were 7 and 9.
The school we enrolled them in in this little hick town arranged for an ESL teacher (but since the area he covered was VERY large, they saw him twice) and they could follow the lessons within 6 weeks. The were put in regular classes from day 1, with regular Canadian children, and it was a very funny challenge for the teachers, who of course did not speak Dutch. It was the combination of sign language, the children's will and need to learn and a lot of humour that helped them all figure it out. We are 5 years along the road now, and when I try to explain something to them and can't find the English words so I use Dutch, it is "English, mom!"

You can just enroll him in Kindergarten, mention he doesn't speak English, and an ESL teacher will be arranged. No worries about it, he is not the first and won't be the last!