He was considered a genius.

After some verbs the direct object can be followed by an ‘object complement’ – a noun or adjective which describes or classifies the object.

• Queen Victoria considered him a genius.
• They elected Mrs Sanderson President.
• We all regarded Kathy as an expert.
• Most people saw him as a sort of clown.
• The other children called her stupid.
• You’ve made the house beautiful.

In passive clauses these are ‘subject complements’; they come after the verb.

• He was considered a genius by Queen Victoria.
• Mrs Sanderson was elected President.
• Kathy was regarded as an expert.
• He was seen as a sort of clown.
• She was called stupid by the other children.
• The house has been made beautiful.

note

For more about object complements, (see here).