You will need a some points invested in the pickpocking perk for this to work!
What you want to do is once you find this guy in the Boston airport save right before your going to do it the very first time just in case he catches you. After you have done that go into his pocket and steal the fusion core. He will then get out and spawn another fusion core for you to steal. Once hes back in the power armor steal the fusion core and rinse and repeat! its that easy.