He is very _, he believes everything.

Explanation

'Fallible' means "capable of making mistakes or being wrong". 'Sensible' means "have done or chosen in accordance with wisdom or prudence". 'Credible' means "able to be believed; convincing". 'Gullible' means "easily persuaded to believe something; credulous".In this case, as the sentence mentions that he believes everything, the adjective closely associated with its meaning is gullible.