it is most desirable that he -- for the information by himself with a few clicks onlineA search B searched C has searched D will search A 为什么?不是虚拟语气吗

不是虚拟，但是固定用法了，只能记。

"he" 的动词不是应该是有es/s?
还有比较式形容词*别不是应该有"the"?
请问有否人顺道解答

这是一个虚拟语气的用法，后面的动词是省略了should
在“It is +adj./n+that从句”中，that 从句常常用“（should)+动词原形”。这些形容词或名词有：strange, surprising, amazing, astonishing, wrong, important, necessary, essential, natural, possible, probable, ridiculous, desirable, advisable, preferable, urgent, insistent, better, funny, pitiful, eager, anxious, unthinkable, a pity, a shame, no wonder等。
类似的句子还有：
It is necessary that some immediate effort (should) be made.必须立即采取行动。
括号内表示should可省略。

it is that 后用原型

楼主你是正确的,这里就是要用虚拟语气,因此原句是：It is most desirable that he [should] search for the information by himself...而因为这个should能够省,省了之后就变成了仿佛第三人称主语直接加原型的假象了....