he didn't feel his words were adequate, needed to increase the volume to compensate;
he knew his idea lacks legitimacy, needed the volume to distract himself from that empty feeling; or,
he wants to intimidate.
Do not get riled up by his behavior. If you get upset, that's what he wants to see, because you've just joined him in sharing the "problem." People may not know what is behind him (or care, in such flux) to get themselves involved. It may be wise to stay engaged, and let people know what is going on.
Some phraseology:
"I am trying to hear what you are saying, but I'm distracted by the high volume."
"I'v been respectful, and I'm sure you want the same."
"I'm sure you care to know, this is the reason..."
(You make him look big by fighting him hard.)
