He will have to do some kind of RE anyway so if he has the interest in and aptitude for PE then why not? Assuming that he is also doing triple science if it is offered, plus the basics? If the RE is being offered as am alternative to PE then I am also assuming that he has already had to choose History / Geography / both?
Someone will no doubt offer an alternative opinion, though (ie that you shouldn't even countenance the idea of him taking a 'non-academic' subject if you don't want him to completely ruin his chances) so I suppose really the best advice would be to canvass the opinion of admissions officers at medical schools as to how in four years' time they will view the GCSE choices potential applicants are having to make now.
As Guest55 says, TSR is full of useful advice in amongst the more 'interesting'