I am a maths teacher in the UK. This is my first year teaching full-time here, so I am not an expert, but I can tell you...
1) There is not an Intermediate Maths GCSE anymore. It's just Foundations and Higher.
2) On the Foundations GCSE, you
can get a C. Higher goes from A*-D and Foundations goes from C-G. If he takes higher and doesn't get an D, he will get a U, but if he takes Foundations, he will only get a U if he doesn't get a G.
Here's some more info:
http://www.direct.gov.uk/en/EducationAndLearning/QualificationsExplained/DG_10039024 In my opinion, not knowing your child at all, if he hasn't been studying the material for the higher paper he will find it VERY difficult and probably get discouraged and perhaps not do as well as he could on the Foundations paper. The material on the Higher paper is MUCH harder than that on the Foundations paper.
You can try talking to his teacher, but if his teacher has said that he doesn't think he would do well on Higher, most likely the school will refuse to enter him for Higher. Ultimately it is up to the school.
If he is really motivated to do better, have him take the Foundations this year, see how he does, and if he wants to improve he can always re-take next year. I know there are many colleges/sixth forms in London that teach GCSE re-takes both at the Foundations and Higher levels. I imagine the same is true where you are. I worked as a supply teacher last year at such a school where many students were taking Higher who had taken Foundations the year before and just wanted to improve and get a B, or even an A.