If he does both "hellenism" wins, and the world is saved!!! HOORAY
More seriously, there is a two fold issue with this:
1) Julian's reforms where primarily academic, and based on Platonism. These kinds of things weren't and aren't Romano-Hellenic Religion, where RHR is traditional ways of devotion, to familial gods and a set of other gods, and some light mystery cults. Julian's largest success was slowing the growth of Christianity, but he didnt stop it, nor did he reverse it. And this is for a variety of demographic and cultural reasons, the largest of which is the higher status of Women in Christianity over any "mainstream" RHR group, i.e. non-Matriarchal Mystery cults. This higher status coupled with the fact that Christianity unlike those mystery cults was "respectable" because it told Women, "listen to your husbands etc. etc. etc." This meant lower class women, many of whom wouldnt even have contact with Mystery Cults, would adopt Christianity as a way to get better conditions in life, primarily getting their husbands to not be so violent or cruel against them. Similarly, Christianity's absolute prohibition on abortion and child abandonment, while not always followed, meant that Christians had higher birth rates, than those in RHR. Against this is arrayed Julian, whose primary attempt was to drive Christianity into the lower classes, and this didnt destroy Buddhism in China, so why would it destroy Christianity in Europe? Similarly unlike Confucianism in China, Julian was a Philosopher who believed that all Gods where created by the Logos, an overgod, thus he attempted to reconcile all the various groups, this inherently didnt work, as many just wanted to do their own thing. He even attempted to change RHR, and paganism in general, the clearest of these being his idea that Priests be model citizens, which was never a pagan idea. In all Julian's religion found little traction for very real reasons, that exist independent of him, and inherent to him, not merely his short reign.
2) Also beating the Sassanids is ASB, no Roman emperor ever beat them, the only group to defeat them was Khalid ibn Walid, and his army of desert nomads.