It's acceptable; because he is allowed to omit the call to prayer altogether while traveling, so it is preferable for him to perform it while riding and then dismount for the iqamah; because if he does not dismount, there will be a gap between the iqamah and the commencement of the prayer due to the dismounting, which is undesirable. See: Al-Durr Al-Mukhtar 1: 260, and Al-Wiqayah p. 140.