Secondary school advise local/bilingual

If you want to stay in English, I'd go for ACE of Brussels, it's however above your max at 14k, but it's your best bet for English quality education in Brussels for a 12 year old in secondary.

A good option is likely to be Le Verseau in Bierges (Wavre). You can get there by public bus from several areas of Brussels and Kraainem too, transferring onto a smaller bus for the last part of the journey. Otherwise parents car share. Otherwise move very near, as the journey from south-east Brussels by car or bus can be long, like an hour. It will cost you about 5k per year. It's a public school with extra fees and it teaches 4/5 hours of fluent level English too per week, about 50% of student body are not French speakers, multi-national, very used to children arriving without French. You'll for sure even find some Ukrainian speakers there. There are always places in the secondary of Le Verseau. The head is British. Your child might be asked to start a year below chronological age or repeat 2nd secondary he'd normally go in to, to get French fluent enough to be able to follow the curriculum.

In Brussels, for a regular French school, I'd opt for Jean 23 Woluwe. They do English immersion, they would perhaps ask your child to start the year below, or repeat 2nd secondary. This school alone does English immersion from 3rd secondary for 11 hours a week. It would be a way of being able to access more of curriculum, they have English mother tongue speakers also in the English immersion. The school is used to children arriving without French at an older age.

Athenee Royal Crommelynck is open to non French speakers too, but the academic level is not the highest there. Would always have places at this school.