At just two weeks old, he weighs less than a lb.
or
At just two weeks old, he weighs less than an lb.
Grammar Checker says "an lb" is right, really?
At just two weeks old, he weighs less than a lb.
or
At just two weeks old, he weighs less than an lb.
Grammar Checker says "an lb" is right, really?
I wouldn't generally use the abbreviation in a sentence. The letters "lb" are a unit and are used with a number. So it is valid to write "1 lb" or "5 m" or "1.4 kg", but you shouldn't write "I'll have a lb of apples". nor would you say "I'll have an ell-bee of apples."
Your grammar checker (and mine) is getting confused by this. It is spotting the error, (a followed by "ell-bee") but it isn't giving you the right solution (change lb to pound)
So the correct spelling should be:
At just two weeks old, he weighs less than a pound.
It is still correct to use the unit with a number:
At just two weeks old, he weighs less than 1 lb.
A general rule-of-thumb:
When the following word begins with a vowel sound, the correct indefinite article to use is an.
In this case, p (from the word pound) is not a vowel sound so, therefore, a is correct.
My grammar checker also says an is correct, this must be a mistake with the grammar checker.