logoped/speech therapist

You do not say which language you are looking for.

If your son is at a local French or Flemish school, there will always be a logopede referral service through PMS / CLB. If your son is not at a local school, then try first your doctor. 

If you wish to obtain part funding from a Mutuelle, your son will need to be tested by a child psychologist who will do IQ tests, also perhaps an ENT specialist to check for any hearing impairment, then the reports get submitted to the Mutuelle for approval for refunds of logopede fees. The alternative is to pay full fees and that will be a minimum of 20 euro per session.