First off, it's normal for kids to gain some weight just before a growth spurt. It's like they need to 'bulk up' for the sudden sprout. Afterwards, their waistlines often thin out again.
If a teen is suddenly gaining weight, it's not a bad idea to have a check up. There may be a physical cause, like thyroid issues. If nothing else, the doctor can reiterate your concerns about healthy dietary choices. It makes a bigger impact coming from a non-family member.
Finally, diets are risky business. You don't want your child to be set up with an unhealthy relationship with food. Continue to limit their access to nutrient lacking foods. If you limit how many potato chips and cookies you purchase, their snack options are much more limited. Just make sure there are plenty of 'good' options available. A healthy lifestyle will serve them much better than any 'diet' that's just waiting to end.