Having looked at stuey’s links:
The paper your son took is marked out of 75. This is the raw mark.
The board sets grade boundaries for this raw mark (these boundaries are tweaked every year to account for slightly harder/easier papers):
51 = A
46 = B
41 = C
36 = D
31 = E
The raw mark is them converted into a UMS mark. What this is out of varies from unit to unit and from course to course. For example, in Edexcel Applied ICT there are three units, each worth 100 UMS marks, to give an overall mark out of 300 for the AS year (but the raw marks are 60, 60 and 90).
Your son’s maths unit is allocated a UMS of 100. Students getting an A in the exam (51 or better) will be given a UMS of 80+, students getting a B in the exam (between 46 and 50 raw marks) will get a UMS of between 70 and 79, etc.
With a UMS of 66 out of 100, your son probably got 44 out of 75 in the exam.
god help is if a C is classed as a pass.
At A and AS level, an E is a pass. You’re getting confused with the GCSE to O-level equivalence where a C at GCSE is the same as an O-level pass.