Why did he say "my son Shlomo is one"?
'Rashi': Even if he were 100 [men], he would not suffice to build it. Further, he is "Na'ar va'Rach"! Malbim - therefore, you must help him!
Malbim: He proved to Yisrael that primarily, Hashem will build the Beis ha'Mikdash. So Chazal (Shemos Rabah 52:4) expounded from "b'Hibanaso" (refer to Melachim I, 6:7:1:1) - it built itself. In natural matters, Hashem always prepares proper means. If the Bayis would be built naturally, Shlomo would have helpers, like Hashem prepared Betzalel and [Ohali'av ben] Achisamach to build the Mishkan 1 !
Surely Shlomo will have helpers. He had 150,000 to carry burdens and quarry rocks, Tzidonim to cut trees, overseers, and Chiram (a talented craftsman)! Hashem could appoint helpers via a Navi after this, or Shlomo himself could choose them! (PF)
Why did he say that Shlomo is "Na'ar va'Rach"?
Refer to 22:5:1:1-2 and the note there.
Malbim: When Hashem acts miraculously, He chooses means that are physically weak and their [spiritual] form is complete, so His help will be according to the Kedushah of their souls. Refer to 29:1:1:2.
Why did he say "veha'Melachah Gedolah Ki Lo l'Adam ha'Birah Ki la'Shem"?
'Rashi': We cannot make it small, since it is for Hashem ha'Gadol veha'Nora, that Shamayim cannot contain Him.
Malbim: The Melachah of building the Bayis will be attributed to Hashem, and not to man. Refer to 29:1:1:2.
