He cannot gain fruition, due to the fact that he is a bodhisatta. gaining fruition will deter him from perfecting the paramis. 2ndly you really do research on this, somehow i do not know how, whenever he reborn in devas or brahma realm, he will recall and remember his bodhisatta vow and will pass away immediately to be reborn in human realm (at times after human realm to animals). 2ndly, sotappana will never be reborn to lower realm below human according to this tradition. so that assumption = 100 percent wrong.
even if that might be the case, but that might be just 1 of the fetters down. due to the vow it might deters him from through the rest of the 2 other fetters. like - he might still hold on to the view of self, and rituals.
he is easily disenchanted with the teachings of Alara Kalama and Uddaka Ramaputta due to his training over the aeon in perfecting the paramis. this condition resulting to the fact that he will surely be enlightened.