Write a letter saying he's bi-polar.

If he's already ill then military service can only make him much worse.

Besides which, no soldier would be happy in combat knowing that his comrades nerve and reliability were suspect - it would be unethical in a military context.

Then there is the question of making mistakes while in the grip of one or other extreme - murder of civilians in a war zone? whilst on a highg feeeling invincible and putting himself and his colleagues in danger, whilst on a low being a deadweight to be dragged around in a stupor of self indulgence ...

First and foremost, someone with Bi-Polar disorder needs to be cared for in a way that the military cannot hope to provide.