SD resident here:

Great ideas for a 15-year-old boy in our fair town:

1. Drop him off at the beach in Pacific Beach, and let him check out all the bikinis without the awkwardness of his parents and grandparents being around. The weather is starting to warm up and should be a target-rich environment.
2. If No. 1 is too edgy, try the USS Midway at the Broadway pier—a super cool way to spend a few hours exploring an aircraft carrier.
3. Belmont Park in Mission Beach
4. Rent kayaks at Mission Bay Aquatic Center
5. Have dinner one night at Punch Bowl Social
6. Comic-Con Museum in Balboa Park

whatever you do, don't fall for Old Town being a place for good Mexican food. It's low-grade slop.