I'm worried I'm losing my son to drugs

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My son is in Form Two in a day secondary school in Nairobi. He has been performing well in school and was formerly a well behaved boy until this year. He has changed and spends most of his time locked up in his room, he does not take care of himself. He has become disobedient and rude, not to mention that some valuable items are now missing from the house. I suspect he is taking drugs but I don’t know how to ascertain this. Please advice on how I can deal with him to put him back on track. He is the firstborn, so I am scared his siblings might follow his footsteps.{Helena}