I say, and with God's success: It is permissible for him to do that; because it is a condition for the validity of the intention in tayammum that he intends a specific act of worship that is not valid except with purity, and the prostration of gratitude is a specific act of worship that is not valid except with purity. Refer to: Al-Durr Al-Mukhtar 1: 165, Al-Radd Al-Mukhtar 1: 165, and Al-Iydah Q6/B, and God knows best.