Question on entering the US
 

I'm travelling to the US with my son who is a US citizen, and I have a B1-B2
visa.
At the arrival airport, can we queue up in the citizen's inspection line or we must go to the tourist inspection line ?

Your son can probably go to either line, but you have to go to the tourist (non-citizen) line.

You are supposed to do one customs form per family, so go with your son in the US line.

I never encountered any issues as a UK citizen (travelling on VWP) accompanying my US citizen wife through the US line

That thing with the non-citizen/visitor line may or may not be the case. Most often the CBP is fine with a US citizen coming in with a non-US relative in the US citizens line. Sometimes they or the line guides don't like it.

It probably depends on the airport and who knows what else.
In our family of five US citizens and one non-citizen we have
been directed away from the citizen queue these last few years.

The arrival airport is Philadelphia and we're supposed to land very early in the morning (5:10AM). Guess if the queue is reasonable I'd just stick to the non-citizen line.
If there is a long queue we can take our chances with the citizens line. worst case they will tell us to go to the other line.