My son getting high off cold medication?

I found Alka-Seltzer plus Day Non-Drowsy Cold Liquid Gels in my sons room. I've hear you can get high off of cold medication. I never thought it could be the Non-Drowsy kind. It contains Dextomethorphan Hydrobromide 10 mg and Phenylephrine Hydrochloride 5mg. If he is getting high, how much do you think he will have to take? So I will know not to give it to him. He is 5 foot and 120 pounds. He is 13 years old. THANKS