Sorry to hear about your experience, though from my years of teaching experience, plus my own three kids, it is not altogether uncommon. With Simon, my second son, I found that I ended up creating my own papers! Basically, I pick out questions that I knew he was good at and copied them onto my own format of paper, and set him 10/15/20/25 minute tests. With each paper, as his confidence grew, I added more and more of the more difficult subjects. In the end as he consistently scored over 75% percent he calmed down.
I should point out that making your own questions up is no longer necessary since there are books out containing hundreds of practice questions of every type (e.g. IPS). Also with so many 11-plus publishers around you can find a format that he can relate to in increasing his confidence, but make sure that the closer you get to exam dates the format used is identical to the actual exam (usually Nfer-Nelson). One point that I read in this website was very good, that is to make sure that above all else the child see the exam as a challenge.
Best of luck.