To turn a MOSFET off, the gate to source voltage has to be zero volts. When you connect the source to +12V, you still have 7V Vgs, when you want it to be off.
To turn it on, you need the gate to be at least 10V more negative than the source. With 5V on the source, you cannot satisfy that condition.
If you need a high-side switch on +12V, you will need a level translator. What you can use depends on how fast the load needs to switch.
Can you use an N-channel MOSFET, and switch the low side of the load? Logic-level MOSFETs in N- and P-channel are available. They will switch with only 5V Vgs.