I'd just say outright "Can you put the gum in the bin when you're done? Thanks." That way you're not telling him off, but ensuring that he knows your preference and making it easy to keep face by obeying you. Or even have a small "gum bin" where they usually dump the gum.

Dumping gum inside the house is just outright rude and disrespectful and if it happens again pull him up on it. He wouldn't do that at home, now would he? (If he would, bigger problem than orginally thought - but most teens are respectful of others' home rules if explained.)

Saying things clearly to teenage boys is a must - the statement above means (a) not in trouble, (b) change of behaviour is needed and (c) change is not optional. ("Could you please... in the future?" means to many teen lads "if you want to do this that's cool, but if not that's cool too.")