Agree with the other posts--
California Baby makes a great 30SPF sunscreen that is THICK and lasts a good while before needing reapplication. It's spendy, but at this point, your son does NOT deserve another sunburn!
Here's an article you can print for his grandmother, send it with TWO bottles of whatever sunscreen you want her to put on him, and a hat with a wide brim to shade his shoulders if possible:
As the summer heats up, overexposure to the sun becomes a concern for outdoor workers. It's not enough just to lather on the sunscreen for protection, says Dr. Craig Eichler, a Florida-based dermatologist. Rather, correct sunscreen application is the key to protecting your skin during this season.
Eichler has recommended sunscreen guidelines for summer skin care, according to a Wellness Junction report. Some guidelines even debunk old myths, such as wearing a tee-shirt while swimming.
Eichler’s sunscreen guidelines include:
Pre-apply: Pre-apply sunscreen at least 20 minutes before sun exposure so it has time to absorb into your skin.
Re-apply: Re-apply every 80 minutes and after swimming or sweating excessively.
Layer properly: If you're going to be wearing insect repellent or makeup, apply sunscreen first so it is the bottom layer directly on the skin.
Be generous: About one ounce of sunscreen (a "palmful") should be used to cover the arms, legs, neck and face of the average adult. "When it comes to sunscreen, don't apply sparingly," says Dr. Eichler.
Understand labels: Look for products with an effective broad-spectrum sun-blocking ingredient, such as Parsol 1789, and a minimum of SPF 15. Products labeled "waterproof" may provide protection for at least 80 minutes even when swimming or sweating. "Water resistant" products may provide protection for only 40 minutes.
Don't use old sunscreen: Check the expiration dates on older bottles of sunscreen. Also, exposure to extreme temperatures can shorten sunscreen's shelf-life, so it's safest to use a new bottle(s) every summer season.
No excuses: If cream sunscreens feel too heavy, try a gel or spray.
Apply under clothes: Many people think wearing a tee-shirt when swimming provides protection, but a wet tee-shirt only has an SPF 4. Be sure to apply sunscreen even under clothes.