When did his followers stop being Jews?
Religions tend not to emerge fully formed from a bottle, so gradually is the best answer. Some pretty quickly, other groups still retained a Jewish identity many centuries later but became increasingly marginalised.
“Jewish Christianity” is a modern term for the beliefs of those followers of Jesus who saw devotion to Jesus as part of God’s covenant with Israel, not as a transfer of God’s promise of salvation from the Jews to the gentiles. Some of them regarded Jesus as a prophet, others saw him as a heavenly power, but all retained their Jewish identity and continued to observe the law. The first Christians were all Jews, but they were not all Jewish Christians by this definition, for they disagreed over the necessity of keeping the law after the coming of Christ. The question of whether gentile believers in Christ should undergo full conversion to Judaism is a highly contentious issue in the New Testament.
Both Paul and his opponents, the leaders of the Jerusalem church, are presented as accepting that gentile Christians did not have to be circumcised or otherwise observe Jewish law (with some exceptions), but whereas Paul, “the apostle to the gentiles,” seems to have been happy with the idea of any Christ-believer abandoning Jewish law, his opponents insisted that those of Jewish origin must continue to practice it. This was the Jewish Christian position...
It was not a stable solution in the long run, and as Christianity spread among the gentiles, the latter became the dominating force. Observance of Jewish law was now forbidden and Jewish Christians were marginalized, to be described by patristic authors of the third and fourth centuries under the names of Ebionites, Nazoreans, and Elchasaites...
Originally, the bastion of law-observing Christianity was the Jerusalem church, the undisputed center of Christianity until the first Jewish war with Rome (ad 66–70). When this war broke out, the Jerusalem Christians reportedly fled to Pella (Ar. Fiḥl) in the Decapolis in Transjordan, and though some returned to the devastated city in 70, they were expelled again after the suppression of Bar Kokhba’s revolt in 135, when Hadrian forbade Jews to reside in Jerusalem. Thereafter, Jewish Christians were concentrated in the Aleppo region in northern Syria, in the Decapolis around Pella, including Dirʿa in the territory of the Ghassānids, and in the Dead Sea region, as we know from Epiphanius (d. 403) and Jerome (d. 420).
Jewish Christianity and the Qurʾān
(Part One) - Patricia Crone
There is a school of thought that Islam emerged out of one of these 'Jewish Christian' sects, although the majority of historians disagree with this view.
There is a similar question to the OP though about when the followers of Muhammad first started to consider themselves as being part of a distinct religion called Islam. In contrast to the traditional narrative, this certainly didn't happen overnight, and likely not in his lifetime.
