He doesn't have to eat it, but if he continues to eat it, it must be done in the sukka, because each eating is forbidden. Especially if its toch achilas pas within the other bread he ate. If its not toch cdei achilas pas, that seems to be a subject of a machlokes achronim, if thats also assur to eat out of the sukka.
Rav shmuel Kamenetsky ( and many others) holds its assur to eat a cbeiza outside a sukka, even more then cdei achilas pas. Rav Nissim karelitz seems to say the mateh Ephraim matirs it.
The latter case also seems to be dependent on this as well..Also it might be assur because it's within your meal, as proved by the shar hatzion 636 siman 2.