It is unlikely that there is a memory difficulty as this only occurs in months. It therefore sounds very much like anxiety which is hampering his performance.
You can try to recreate the exam environment at home prior to the exam by giving him a home set paper of a similar length to that of the real exam, keep everything very quiet, don't allow him to get up etc. By doing this and keeping the questions initially to those you know he can cope with, the anxiety is likely to lessen as he sees his good performance in a usually stressful situation.
This may take a number of trials to succeed; I do not recommend doing this in the afternoon after all his homework but perhaps every second weekend in the morning.
Rescue drops can also assist with anxiety and are natural; however, it would be best to try remedy the situation without medication and rather teach a coping strategy he can use his entire life.
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