Using P-channel FET as switch
Personally, I find the most reliable method of switching a PMOSFET is by using an NMOSFET. The circuit I generally tend to use is like this:







simulate this circuit &ndash; Schematic created using CircuitLab

With this circuit, R1 is pulling up the gate of M1, keeping it off. R2 is pulling down the gate of M2, also keeping it off. Once the 5V is applied to the gate of M2, it will pull the gate of M1 low, which will allow M1 to enable the Boost_EN pin.

This is a circuit I have used many times and it has always been reliable. 

Edit due to addition of circuit in original question

The original wording of the question sounded like the EN pin was needing to go HIGH when 5V was applied. Now it seems that the intention is for the pin to go LOW when 5V is applied. This simplifies things. Replace M1 with a pull-up resistor:







simulate this circuit

With this circuit, your EN pin is always pulled HIGH by the resistor R1. When 5V is applied, M2 will pull the pin LOW.