Learn Italian: Questo ragazzo è povero, ma è meno debole di Luca. - This boy is poor, but he is less weak than Luca.

To say someone is "less [adjective] than" someone else, Italian uses the pattern meno + adjective + di. In this sentence, meno debole di Luca translates directly to "less weak than Luca".

Since we are talking about a boy (ragazzo), why does debole end in -e instead of -o?

While many adjectives have separate masculine (-o) and feminine (-a) endings like povero in this sentence, some Italian adjectives end in -e for both genders in the singular. Debole (weak) is one of these, so it stays debole whether describing a boy or a girl.

Why isn't there a word for "he" right after ma (but)?

In Italian, you can usually drop subject pronouns like lui (he) because the context makes it clear. We already know the sentence is about questo ragazzo, so saying ma è (but is) is the most natural way to say "but he is".

Could I use quello instead of questo here?

You could, but it would change the meaning. Questo means "this" (someone or something close to you). Quello means "that" (someone or something further away). Here, questo ragazzo means specifically "this boy".