How to approach?

[mimi]

I don't think you can change what these parents are choosing to feed their child. I am sure their family doctor is aware of this child's weight issues and has already advised Mom who may not be following doctors advice but is well aware of what he should be eating. All you can do is feed him well at your home and to help him want to make healthy choices by teaching him the benefits of doing so.
Explain your food choices to him like juice tastes good, but it has a lot of sugar which our body doesn't like so we chose to eat our fruit instead. Just encourage his love of the fruits and vegetables and congratulate him on his good choices.