APs operating in the same chunk of the spectrum can utilize it better if allowed to use the entire spectrum due to collision avoidance algorithms such as CSMA/CA, as opposed to dividing the spectrum into smaller chunks and having each AP use their smaller chunk....Could someone please explain why he is or isn't correct?

A lot depends on your definition of "chunk." As a practical matter, you only get two 40 MHz channels in the 2.4GHz band, and it's more susceptible to interference than a 20 MHz one. The number of adjacent APs, number of clients, and types of traffic will all affect performance and may make narrower channels work better than wide ones.

A better question is why use 2.4 GHz at all? Any device capable of 40GHz on 2.4 GHz is almost certainly capable of using the 5GHz band, where there are plenty of channels and less interference. But perhaps there is a justifiable reason that we don't know.

However, since he is the senior network engineer we have to do things the way he says

This is the bottom line, and I doubt that anything that anyone here says will change his mind. You can rest easier that your opinions are just as valid as his.

APs operating in the same chunk of the spectrum can utilize it better if allowed to use the entire spectrum due to collision avoidance algorithms such as CSMA/CA, as opposed to dividing the spectrum into smaller chunks and having each AP use their smaller chunk....Could someone please explain why he is or isn't correct?

A lot depends on your definition of "chunk." As a practical matter, depending where you are, you only get one 40 MHz channels in the 2.4GHz band (US), and it's more susceptible to interference than a 20 MHz one. The number of adjacent APs, number of clients, and types of traffic will all affect performance and may make narrower channels work better than wide ones.

A better question is why use 2.4 GHz at all? Any device capable of 40GHz on 2.4 GHz is almost certainly capable of using the 5GHz band, where there are plenty of channels and less interference. But perhaps there is a justifiable reason that we don't know.

However, since he is the senior network engineer we have to do things the way he says

This is the bottom line, and I doubt that anything that anyone here says will change his mind. You can rest easier that your opinions are just as valid as his.