One synonym for "give him his due" is to acknowledge or recognize someone's effort or contribution. Another synonym is to give credit where credit is due. It means to give praise or recognition to someone who deserves it. Other synonyms for "give him his due" include to give someone their fair share or to acknowledge someone's accomplishments. Additionally, you can use the phrase "give someone credit where credit is due" which means the same as "give him his due" and involves acknowledging someone's contribution or achievement. Overall, there are many ways to express the sentiment of giving someone their due credit or recognition.