Can my 16 year old drink pediasure?

Yes. It is just another form of nutrition. You can achieve a similar nutritional benefit with Carnation instant breakfast packets added to whatever milk product he likes for a fraction of the cost. It is available in vanilla & other flavors.

Your 16 year-old should learn to have a balanced mixed diet. For healthy diet: http://www.choosemyplate.gov/., exercise 30 minutes/day, drink enough water daily so your urine is mostly colorless, have safe sex, no tobacco alcohol weed or street drugs. Get HPV and Covid vaccines. Wish you good health! Safe sex: https://www.hopkinsmedicine.org/health/wellness-and-prevention/safer-sex-guidelines

I agree, but if there is a concern regarding weight loss, a visit with your Pediatrician is indicated to review systems and to ensure there is no underlying condition. This may include an eating disorder, chronic infection, or GI problem.