No, the phrase "Rejoice rejoice sing loud - give thanks" is not iambic pentameter. Iambic pentameter consists of lines with five feet, where each foot has two syllables with the stress on the second syllable.

Philippians 4:4 - Rejoice in the Lord always. Again I will say, rejoice! [NKJV]

Rejoice is a verb.

I don't think it matters, but the first one is the old way to say the same thing. The 'alway' is the KJV usage and the addition of the 's' is how we say it these days. The meaning from the original Greek is entirely the same.

The day has come, the day for which I've wept and prayed;

The time to shout- my joy has now arrived;

My son's come home- his face on the horizon, and

My joy is full because he is alive-

Alive! Alive! Alive!

Rejoice! Rejoice! My son is coming home again,

Rejoice! Rejoice! Go- kill the fatted calf;

Rejoice! Rejoice! My son is coming home again,

Rejoice! Rejoice! Rejoice on his behalf.