Power NMOS used as voltage switch
You have gotten great feedback already. It's obvious that the best approach is to use a PMOS instead of the NMOS. You could use something like this, with a PMOS (and NMOS) to get what you are looking for:







simulate this circuit &ndash; Schematic created using CircuitLab

With this you can use a low voltage signal (e.g 3.3V, 5V) to turn on the PMOS. With no signal at the NMOS gate, both the NMOS and the PMOS are OFF. When you apply 3.3V at the gate of the NMOS, that will turn it ON, and will pull the gate of the PMOS down to 0V—this will turn ON the PMOS since now VG-VS (e.g -5V, -12V) is enough.

Just need to pick the two the MOSFETs. For the NMOS, something like the DMN3067LW works great (turns on with low voltage). For the PMOS, you need to pick one with the 5A current capability you want.

One caveat though, if you were to keep the same configuration as you have it, you need to pick a PMOS capable of turning on properly with 3.3V, and 5V. The 8V regulator should work fine because you have the PMOS before the regulator. So you have a high voltage signal (12V), and when you pull the gate down to 0V, VG-VS=-12, enough to turn on most high current transistors (and with lower RDS(on)). 

In your 3.3V and 5V regulators, you have the PMOS, after regulation which puts the source either at 3.3V or 5V which means that pulling down the gate will have a VG-VS of either -3.3V or -5V, enough to turn on the PMOS but possibly with higher RDS(on) and at 5A that maybe a drop you would not want (plus heat). For those, I'd place the PMOS/NMOS control circuit before the regulators (just like for the 8V regulator) so that there is enough drive (approx. -12V). You could find transistors that will turn on for 3.3V or 5V but at the expense of higher RDS(on).

Hope this gives you more insight.