Please Help.
How Am I Related To My Nan's Sister's Grandson?

Thanks

x.

If by nan you mean grandma then I think that is second or third cousins.

I think you would be third cousins. Your parent would be cousins with the grandsons parent... then you would be second cousins with the grandsons parent making him your third cousin

NAN / NAN'S SISTER

YOUR PARENTS (cousins) / GRANDSONS PARENT

YOU (second cousins to Nans sister's grandsons parents)

YOU ( third cousin to the grandson) / GRANDSON

Hope this helps