I understand your concern for your teenage son's nutrition and weight loss. Here are some suggestions for protein shakes that can help him replenish his energy and nutritional intake:

Whey protein powder: Whey protein is a popular choice for athletes and fitness enthusiasts due to its high-quality protein content and quick absorption. Look for a whey protein powder that is low in sugar and additives, and consider adding fruits, vegetables, and nut butter for added nutrients and calories.
Plant-based protein powder: If your son prefers a vegan or lactose-free option, plant-based protein powders can be an excellent choice. These powders are often made from pea, rice, or hemp protein and can be combined with other ingredients to create a balanced shake.
Meal replacement shakes: Meal replacement shakes are designed to provide a balanced meal in a liquid form, containing a mix of protein, carbohydrates, and fats. These can be helpful for individuals with a limited appetite or who struggle to consume enough calories. Look for a product that has a good balance of macronutrients and minimal added sugars.
Smoothies: You can create a nutrient-dense smoothie by combining fruits, vegetables, protein powder, and healthy fats like avocado or nut butter. This can help increase the calorie content and provide a well-rounded meal replacement.

When selecting a protein powder or meal replacement shake, always read the label to ensure it contains high-quality ingredients and minimal added sugars or artificial flavors. Additionally, consider consulting a nutritionist or healthcare professional for personalized advice based on your son's specific dietary needs.

Here's a template for a protein shake recipe you can try:

1 scoop of protein powder
1 cup of milk (dairy, almond, oat, etc.) or a dairy-free alternative
1 banana or 1 cup of frozen berries
1 cup of spinach or kale (optional)
1 tablespoon of nut butter or avocado (optional)
1 tablespoon of chia seeds or flax seeds (optional)

Blend all ingredients until smooth and serve. Feel free to experiment with different ingredients to find a combination that your son enjoys