Schools

Tough one. If he’s prepared to go for it, he can learn Dutch in 6 months and will be very competent in a year. Education in Flanders is very good. If you can afford an intensive language school, it’s amazing what can be done in 3 to 4 weeks. But you need to go straight to the country immediately you finish and use it daily. I talk from experience.
I believe Liege has an international anglophone school but that won’t help getting local friends and you’ll need transport and a healthy bank account or employer who pays.

Stick him in any school and ask then for whatever help they can give.
The help is there.

Schools in Flanders are generally equipped to deal with incoming students without Dutch as it is a regular occurrence.
His teachers will have English and, as a teenager, his classmates will also have a basic knowledge from school, social media, music and TV (English language programmes are subtitled on Dutch language channels).
I've lived in small town Flanders for 25 years - the last 15 in a town almost identical in size to Tongeren - and have never had a language problem. Professional people all speak English and most of the general population have some level of the language and they don't want to know how well I speak Dutch; they want to practise their English.
Close friends moved here from Denmark. Their son faced starting in school where not a single person spoke the only language that he knew. Within 6 months he was managing OK and within a year he was at the same level as most of the rest of the class.
If you are moving to Tongeren you must have contact with someone here. Can you not ask for a contact with someone living there who can give you the benefit of their own experience?
Good luck.

There is no international English school in Liege.

You could consider first an OKAN class and then transfer to ordinary class. OKAN is the name for "Dutch for Newcomers" class in the Flemish education system. You find them integrated into Flemish schools.

The nearest Flemish secondaries to Tongeren with OKAN classes are 3 schools in Hasselt and 1 school in Sint Truiden. You'll find them by looking on this list, clicking on "Limburg", then going to the bottom of the page and clicking on "toon de kaart" and you can see them geographically from Tongeren.

https://data-onderwijs.vlaanderen.be/onderwijsaanbod/lijst.aspx?hs=311&s...

You can also just consider a secondary without OKAN and ask for extra help. You can see the list for Tongeren on this list. Go to drop down list and choose "Tongeren" and you'll see the list.

https://data-onderwijs.vlaanderen.be/onderwijsaanbod/lijst.aspx?hs=311

You can also just

If you are moving here permanently, or you know it will be 4-5 years, then it will work. However, be aware that even the brightest kids will take at least 6 months to begin to "manage" in the language, and don't be at all surprised if they need to repeat at least one year of school.

"Changing" languages is possible, but inevitably there will be an impact on your child, socially, emotionally, and educationally for a minimum of at least 6-12 months before they catch up.

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