Okay at 17 he is still growing. Also assuming he is at normal height, 120kg isn’t hopeless. Personally I don’t think he should really fast yet.

Here is what I think he should do.

1) Cut all refined added sugar out of his diet. That pretty much eliminates all processed food. Replace sugary foods with whole fruit, vegetables, nuts and seeds. I think diary is fine as long as it isn’t ice cream, desert or a milk shake. He will have to study a bit here, sugar is a very sneaky additive.

2) Eat slowly and stop when he starts feeling full. I can’t stress how much his has helped me not to overeat. I learned from my youngest daughter who has always tended to be under-weight.

3) If he wants to fast, focus on no snacks after supper until after breakfast. Give himself around 12 hours break from eating everyday.

4) Push up his activity. Strength training is great for that age!

His body will probably work with him. If he still needs to lose a lot in a few years at least he should be fully grown then.

Fasting has profound impacts on the body and it shouldn’t be taken lightly. I have a 16 year old, I wouldn’t allow him to fast until he is full grown. It might be okay for some reason rather than weight loss, but for just burning fat, teenage bodies are built for that! He just needs to start eating healthy and being active.