The regulations covering remote access include the following:
"5(3). Those conditions are that the member in remote attendance is able at that time--
(a) to hear, and where practicable see, and be so heard and, where practicable, be seen by, the other members in attendance,
(b) to hear, and where practicable see, and be so heard and, where practicable, be seen by, any members of the public entitled to attend the meeting in order to exercise a right to speak at the meeting, and
(c) to be so heard and, where practicable, be seen by any other members of the public attending the meeting."
So the question is, what does "where practicable" mean. It must take into account the fact that some people do not have the correct IT setup to be seen, including the facility in Zoom to access a meeting by telephone. Wider interpretation obviously hasn't been tested in the courts. My advice would be to ask the councillor why it is not practicable for him to be seen. If it is a matter of personal choice, members might decide to include reference to this fact in the minutes to record the fact that there was no practicable reason for the councillor to be invisible to the meeting.