“Now, my son, the LORD be with you, so that you may succeed in building the house of the LORD your God, as he has spoken concerning you.
Though Solomon was young and tender, he was capable of receiving instructions, which his father accordingly gave him, concerning the work for which he was designed. When David came to the throne he had many things to do, for the foundations were all out of course; but Solomon had only one thing in charge, and that was to build a house for the Lord God of Israel, Ch1 22:6.
Commenting on 1 Chronicles 22:6-16
Now, my son, the Lord be with thee, &c. Or "shall be with thee" (r), as some; and if it be as a prayer, it was no doubt a prayer of faith; the Targum is,"may the Word of the Lord be thine help:" and prosper thee; may success attend thee: and build the house of the Lord thy God, as he hath said of thee...
Now, my son, the LORD be with thee; and prosper thou, and build the house of the LORD thy God, as he hath said of thee. (e) He shows that there can be no prosperity, but when the Lord is with us.