Grandchild not eating well

Hi, My Grandson of 4 yrs is refusing to eat the majority of dinners offered.
He has a 2 year old brother & sister who are twins, so a meal if decided upon & there is nothing else.

1. My 4 yr old grandson attends school 4 days a week a week & is given school dinners, he doesn’t always eat it all
2. A nanny is employed as parents work, she cooks evening meal & gives my grandson the same portion size as the 2 yr old twins, it is meagre
3. My grandson has lost lots of weight, is not sleeping well & is having negative tantrums
4. His parents appear reluctant to take it seriously & seek medical help, even though this has been going on for over a year

I want to feel I can gently suggest taking him to see a GP, but afraid to do so in case I am seen as interfering & I don’t want to lose a good relationship with my Son & his Wife.
This is tearing me apart, I do think if a grandparent is concerned for their grandchild’s health they have a duty to speak up, my husband says
“ stay out of it”
Any thoughts if you were in my position