Learn Russian: Сын похож на брата. - The son looks like his brother.

In Russian, 'looks like' is expressed using the short-form adjective похож, not a verb. Because the subject сын (son) is masculine, we use the masculine form похож. For a feminine subject, you would use похожа.

Why does the sentence use the preposition на?

The adjective похож always pairs with the preposition на to mean 'looks like' or 'is similar to' someone or something. It is a fixed combination you just need to memorize.

Why is the word for brother spelled брата instead of брат?

The phrase похож на requires the following noun to be in the Accusative case. Because брат (brother) is a living, animate masculine noun, its Accusative form takes the ending -а, becoming брата.

Why isn't there a word for 'his' before brother?

Russian often leaves out possessive pronouns like 'his' or 'my' when the context makes it obvious. Here, it is naturally understood that the son looks like his own brother, so extra words like своего or его aren't necessary.