On your second trip to the Citadel, travel to the embassies and then immediately head left. Ambassador Osoba can be overheard trying to find information about his missing son. Speak to him and you will find out that his son's squadmates returned from Benning without him but will not talk about what happened, and you can offer to help.
During the N7: Cerberus Abductions mission, look for dogtags which are located in the courtyard opposite building number 27, next to the closest square column to building number 22. Look carefully as it is easily missed.
If you miss the dogtags during N7: Cerberus Abductions you can purchase the dogtags from the Spectre Requisitions store in the Spectre Office at the Citadel Embassies for 1,000 credits and then present them to Osoba. You gain 5 Reputation and 90 experience for completing this mission.
Note: This mission must be completed before completing Priority: Tuchanka as it will be the last opportunity for the player to visit the Citadel before Priority: The Citadel II, after which Ambassador Osoba will no longer be available.