You can do some things.. but he has to do the rest!
You can be supportive in a relaxed way..make sure he has a decent quiet place to study, provide his food and check he has anything he needs. You can also do gentle nagging..but that's about it I'm afraid!
Is he yr 12 or 13? A lot of very bright students who sailed through their GCSEs come BADLY unstuck at AS level. I have two who have sat A levels in the last two years and have seen it over and over again (including my own DD2) . The jump from GCSE to A level is BIG and in particular the sciences and maths can be a real blinder for those who found it easy before. My DD2 did badly in one of her AS levels (she had Glandular fever for 4 months which really didn't help, but she also didn't work hard enough) and it was a massive wake up call.. she pulled her socks up and miraculously has just jumped two grades in the resit but it's been a hard slog.
Is he doing the right subjects for him? Quite a few students I know realised they were not doing the right subjects and restarted the year again.. it's a long two years if you are doing something you aren't enjoying at all.
Does he JUST panic in the actual exam, or is he panicking because he isn't well enough prepared? For pure nerves maybe rescue remedy, or maybe even visit the gp and see if he can have something to calm him down.. however if it's panic because he isn't prepared well enough, then he needs to get his study prioritised.
One thing we have done was to get a private tutor for my DD1 for the subject she was doing badly in (if he's doing bio I can give you a link..she does it online via skype and it has been fantastic!). Again we found that several of my DD1s friends are having tutors for weaker subjects.. that extra concentrated 1:1 support has made a huge difference.. £22 a week but worth it!
In the end, if he wants to go to University, he will HAVE to be able to perform in exams..be prepared etc because that's what continues at University! It may be that if it's not lack of preparation related, or being in need of a bit of help, it might be wise for him to look at all the other options out there for him...there are may routes to success, not just a degree:)
I totally understand your worry.. been there twice!