From my reading of quite lot of past EP reports, issues like this can come down to Self-Esteem (a mismatch between expectations and self-belief) and Locus of Control (feeling in control of one's own destiny.)
I think you might be wise to consider finding a counsellor who works with adolescents. It may be that he is "a pressure cooker" who needs someone impartial to vent to.
I am surprised at the Head suggesting the EP route because it could "medicalise" and even worsen the situation.
However, if you are determined to go that way, there is quite a sophisticated Search facility on the British Psychological Society website, here: http://www.bps.org.uk/psychology-public ... ychologist
You could, for example, narrow your search to "Adolescent, Mental Health, Academic Performance".
I would then call a few reasonably local EPs from that search list and put the issue to them. I would expect them to be willing initially to discuss the issues with you over the phone, and I would hope to hear mention of words similar to those I have used above.
I would also expect them to tell the type of tests that they would use to discern what is going on with your son. If you then let us know what they say, we may be able to tell you how appropriate the tests would be.
The one thing you do not
want in this situation is an "off the peg" Educational Assessment because it might leave your son feeling under even more pressure or undermine his self-esteem even more. Run a mile from anyone who seems to be suggesting that.
Edited to add: Lots of cross-posts while I made my coffee.