There is no such thing as "will be send". Passive always requires Past Participle, so look out for that. The Past Participle of "to send" is "sent".

Which is correct to be send or to be sent?

The past participle of "to send" is "sent." So he knows who sent him this amazing gift! This is because the phrase "who sent him this amazing gift" is a clause which requires a finite verb. Specifically, it contains the simple past tense of "to send," which is "sent."

Would be sending meaning?

"I'll be sending you" means that you will be in the process of sending, but does not specify that you complete that action; it is continuous or ongoing. For example, if you are going to send someone a single set of photos, you might say "I'll send you the photos".