No, he remains a senator, and actually becomes supreme chancellor after the death of emperor palpatine
No he doesn't. Although there is no canonical mention of Jar Jar after Revenge of the Sith, It is likely he stays a senator. After the Emperor disolves the senate in A New Hope, he probably returns home to Naboo. After Palpatine dies in Return of the Jedi, and the New Republic is set up shortly after, the first Chief of State is rebel leader Mon Mothma, followed a few years later by Leia Organa Solo.