ولد مريض ضعيف = A sick, weak boy. In Arabic, the adjective must follow the subject/noun in definition, they're both definite, or both indefinite, that's why since the subject "boy" is indefinite, all the indefinite nouns after it are adjectives. In the first sentence, the "boy" was definite, so since the word "weak" was indefinite, it became the predicate, not an adjective. To simplify: For the first sentence: الولد = definite. المريض = also definite = adjective for "الولد". ضعيف = indefinite = predicate. For the second senence: ولد = indefinite. مريض = also indefinite = adjective for "ولد". ضعيف = also indefinite = adjective for "ولد".

Both of the two sentences are correct to translate as ولد مريض ضعيف but when you translate the Arabic sentence to English both of them works while - a sick boy is weak - is في الإعراب more correct.

أصل الجملة الثانية ٌهذا ولدٌ مريضٌ ضعيف أو هو ولدٌ مريضٌ ضعيفٌ لست متخصصا في اللغة ولكن هذا ماظهر لي .. والله أعلم

A sick, weak boy

المريض : adjective