Is it obligatory for an ordinary person to stop listening to the shaykh's lesson in the mosque until he knows whether the shaykh is endorsed or criticized?

The default is that a Muslim is sound (in faith and practice). A Muslim listens to admonition in the mosque from a preacher whose state is concealed, unless he says something that is surprising or whose correctness is doubted, in which case one should ask the people of knowledge. As for someone a Muslim regularly associates with for seeking knowledge or a fatwa, he must be trustworthy in his knowledge, piety, and religion.