Learn Portuguese (Portugal): O cão tem fome. Alimenta-o agora!

Because alimenta is an affirmative command. In European Portuguese, when giving a positive command, object pronouns like o (meaning him or it) must attach to the end of the verb with a hyphen.

Why is it alimenta-o and not alimenta-lo?

The pronoun o only changes to lo when the verb it attaches to ends in r, s, or z. Since the informal command alimenta ends in the vowel a, the pronoun stays as o.

How would I change this to a negative command, like "Do not feed him now"?

You would say Não o alimentes agora. When you make a command negative, the word não acts like a magnet and pulls the pronoun o in front of the verb. Note that the verb also changes to its negative command form, alimentes.

Why does the sentence use tem fome instead of a word that translates directly to "is hungry"?

In Portuguese, we express hunger by saying someone literally has hunger using the verb ter (to have). This is a common pattern you will see for other physical states too, such as ter frio for being cold or ter sede for being thirsty.