Eating problem - 4 year old; joint custody?

My son has shared custody of our 4 year old grandson. When with his mother meals consist of pop-tarts and junk food. When with our son, he refuses to eat anything but pop-tarts. When give the option of dinner or bed, he willingly opts for bed even though it's 5:00pm... His mom has no regard for what he eats as long as he's out of her hair. Any help?