One BTEC at GCSE level is not going to hinder anyone's chances of getting into university. Doing a BTEC instead of a-levels may limit university/course choice a little bit (most universities will accept BTECs provided they are appropriate to the course and the right modules have been taken) , but at GCSE, it really won't make any difference.
Most universities will be interested in grades rather than the specific option choices taken. His maths grade may well be important, as will his English grade. To avoid limiting university choice, he would need at least a B in English language (although this can be re-taken alongside A-levels). Overall, the better his GCSE grades are, the stronger his application will be, and some good universities for engineering, like Imperial, do place emphasis on having good GCSEs.
Subject choice only really becomes important post 16.