^^^ this, about the cat nap and early bedtime.
It might seem crazy to take away an opportunity for him to sleep but it's really important at this stage to re-train his sleep expectations, so that he knows he doesn't get another go at it later in the day and will therefore sleep better earlier. So instead of a CN, go for an early bedtime. Early bedtime can also really help with early wake-ups (later bt can often mean an early WU, babies are so strange...). By doing no CN and an EBT, you're teaching him to sleep more at night rather than during the day ... which is where we want to end up.
What I hear from your posts, Mandy, is that you're really afraid of him getting OT. This is a good way to be and all credit to you for looking out for your boy! Right now he needs you to push him a little bit and help him establish a better routine. You're essentially re-teaching him his routine. You need to be mega-consistent for a few days so he doesn't get confused. That means, you need to try an A time and stick with it, and avoid catnaps, if you can at all, for three days at least. Does that sound possible?