Tongeren is connect by train to Hasselt, so I'd look at the Hasselt schools as a first option. 22 minutes by train.
OKAN lasts 1 to 2 years and with a 13 year old, I'd expect 2 years. It is intensive Dutch with slow integration into regular classes. You won't get a better option than this.
www.vlaanderen.be/nl/onderwijs-en-wetenschap/onderwijsaanbod/onthaalonderwijs-voor-anderstalige-kinderen-okan
You have 3 secondaries with OKAN to choose from in Hasselt, none in Tongeren - though there is a 2013 link to a school in Tongeren, it does not appear on the official Flemish education website and does not appear on the school website, so assume the proposal to start it in Tongeren in 2013 never happened.
You might even want to consider the 13 year old staying on at the same school, post OKAN class, so take a look at the offerings of that secondary.
Flemish secondaries offer 4 types of education from age 14-18, those years are also known as "2e graad" 14-16, "3e graad" 16-18. For the 12-14 "erste graad" it's A or B, but your 13 year old will miss this if in OKAN, as that is a specialism. It's therefore important to look at what happens from 14-18 when schools specialize.
data-onderwijs.vlaanderen.be/onderwijsaanbod/default.aspx/so/structuur
Some schools do 1 type of the 4 types, some do 2 or 3 types. They are called ASO= general academic, TSO = technical (still to lead to university), BSO = professional (tends to lead to a trade like plumbing), KSO = talented arts and that is speciality and quite rare! You are most likely to be looking for ASO and TSO secondaries.
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Campus Hast. Does TSO and BSO - very near train station
data-onderwijs.vlaanderen.be/onderwijsaanbod/adres.aspx?sn=39107
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Provinciale Handelschool Hasselt. Does ASO, TSO, BSO - not so near train station but plenty of buses from train station, it's in the area with the 2 universities and other secondaries, swimming pool, ice rink, lots to do around the school! It's probably the school out of the 3 I'd ask at first, given it does ASO and TSO too.
data-onderwijs.vlaanderen.be/onderwijsaanbod/adres.aspx?sn=39263
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Go! Next KTA 2 - does TSO BSO
data-onderwijs.vlaanderen.be/onderwijsaanbod/adres.aspx?sn=44156