The Health Visitor is involved and will be very aware of your DGS progress and your DiL's concerns in addition to being up to date with current advice and options available to parents.
Rest assured no Health Visitor wants a baby/mother in their care to end up in a Crisis situation and will therefore be taking action to prevent that.
From what you have said about your DiL having her milk checked and the HV suggesting a change in your Dil's diet it is clear that HV your DiL and your DS are fully aware and taking action.
The action being taken are not the actions you suggest and you may feel therefore that too little or nothing is being done to address your DGS situation.
I can understand how the situation can be a cause of worry for you but in your worry I wonder if you are missing the actions that are being taken to improve your DGS ability to thrive.
Your DS and DiL are the parents in this situation and your repeatedly voicing your concerns, will appear as criticism, it is showing a lack of trust on your part of their abilities.
Repeating your concerns and solutions, when they have clearly informed you that solutions and actions are taking place to ensure your DGS care and well being will not help your present and an ongoing relationship with these parents.
Please find a way to trust your DS, DiL and the HV.
Worry is a difficult things to deal with as is may have a rational basis and irrational basis, sometimes it's difficult to know whats rational and what's not and in the end worry is worry and robs us of some of our peace.
You may find researching the information your DiL is working with helpful as then you will also be informed and up to date with modern thinking of the actions being taken for your DGS well being.
I have to say my DD2 was lactose intolerant from birth, no one knew. We, her and I, struggled with breast feeding, her vomiting and not gaining weight.
We, the HV, GP and breast feeding specialist, tried formula and cows milk. It was only when she was 6 months old that Lactose was suggested as the problem. By then she was starting to wean and once her reliance on milk was reduced she was away and was a very healthy robust child.
She is now 40 so it isn't new and it's not a fad.
Fortunately the understanding Lactose, among other factors affecting babies feeding have come a long way and it is better understood that formula unless very specialised may be the worst thing for a Lactose Allergic/Intolerant baby.
I'm sure and at some level you too are sure that your DS and DiL won't let your DGS fail to thrive without doing everything they can to care for him.
If you can find your trust in your DGS parent to take care of him, even if the care differers from how you would delver his care, then may be your worries will lessen.
This being a GP isn't for the feint hearted :-)
Hang in there, bring your informations and understanding up to date with theirs and then give them praise for the care they are giving your DGS.
Best Wishes to you all for the best possible outcome.