Hi, and thank you for coming to our site with your question.
This is, unfortunately, a common occurrence in situations such as the one with your grandson.
There are centers that specialize in dealing with children in cases like this, and over the course of a month to six weeks of either inpatient or intensive outpatient work.
I am not familiar with specific ones in the UK, but what you would likely be looking for (asking the doctor for suggestions) is a "feeding center".
I wish you all the best of luck and hope he grows up to be a happy and healthy adult.
Please let me know if you have further questions.
I'll be leaving the computer shortly, but plan to be back on in the morning.