How will he make him Brachos?
Radak: This is like "ve'Hye Brachah 1 " (Bereishis 12:2).
Malbim: When he wanted to build the Beis ha'Mikdash, and the promise came via Nasan ha'Navi that kingship will never leave his house - "va'Hakimosi Es Zar'acha;
Radak (Bereishis 12:2): His Brachah and success will be so great that he will be a Brachah for others. When one blesses his son, he will say 'Hashem should bless you like the Brachah of Avraham.'
Why does it say "la'Ad"?
Radak #1: Even after his death they will bless via him, like "Becha Yevarech Yisrael Leimor Yesimcha Elokim k'Efrayim vechi'Menasheh" (Bereishis 48:20).
Radak #2: This refers to his seed. Just like people bless through him, they will bless through his seed, like it says about Avraham "v'Hisbarachu v'Zar'acha" (Bereishis 22:18).
What is "Techadehu"?
Rashi: It is an expression of Chedvah (joy). Radak - this is like "va'Yichad Yisro" (Shemos 18:9).
Why does it say both "Techadehu" and "v'Simchah"?
Radak: This is because the Simchah is so great. Also "u'Tevo'ach Tevach" (Bereishis 43:16), "u'Ma'alah Ma'al" (Vayikra 5:21) and similar verses (double the expression due to the intensity).
What do we learn from "Es Panecha"?
Rashi: In Gan Eden. Those who expound this to discuss Melech ha'Mashi'ach brought a proof. [It says about Mashi'ach] "v'Ad Atik Yomaya Meta u'Kedamohi Hakrevuhi", and "v'Hikravtiv v'Nigash Elai" (Yirmeyah 30:21) 1 .
Radak: His great Simchah will be when You put the radiance of Your countenance on him. He will not rejoice in his great army, gold or silver, only when You turn to him, like "u'Fanisi Aleichem" (Vayikra 26:9). The light of His countenance is when he gives to man success in everything that he does.
Malbim: His Simchah will be "Es Panecha" - when he sits in front of Hashem.
Shocher Tov does not explain how we know that our verse discusses Mashi'ach. Perhaps it is because the other verses cited discuss bringing Mashi'ach close to Hashem, i.e. "Es Panecha" (PF).
