*hugs*
I think the problem is that MPI is not common enough for the average GP or HV to know very much about it
The chief HV in my area (who made me cry after most visits
) told me that Aptamil Comfort is the same as a prescription hydrolysed protein formula - IT ISN'T!! - makes you wonder if they have to fail an IQ test...
Although they always caution against it, I think you can learn far more online in places like this than you can from HVs or GPs. Even the 'experts' the peds don't always keep up with the latest information... ours told us there is no benefit to BF after 6 mo and that it's positively harmful after 8 mo
(No I don't agree!)
Unfortunately it looks like you'll need to educate your GP
Just ask them to do a quick search online for MPI symptoms and they'll soon see that it is very likely your LO may have it.
Good luck, really hope you get somewhere this time