"he is supposed/about to attend the **** conference in November this year"
Please help me to write this sentence correctly and in a formal way.
You have two different sentences here.
"He is supposed to attend..." means that there are people who expect him to attend, perhaps because his job requires him to, or perhaps because he has said he would attend. "He is about to attend..." means that the conference will take place very soon, and he is going to attend. In other words, his departure to attend the conference is imminent.