Mosfet won't turn fully on
Your problem is it is not configured as a switch. It is configured as a source follower. To make it switch fully on, you need the gate to be some volts above the source. Since the gate is held at about 0.5*VBAT by the voltage divider R7 and R8, anything that tends to drive the source above this level will start to turn "off" the FET.

This will have roughly the same effect as a BJT emitter follower, although the gate-source drop is harder to predict than a BJT's base-emitter drop. The voltage at VBUS will be a little bit below 0.5*VBAT, which is exactly what you see.

If you want to use it as a switch, you have a few options:


Move the NMOS FET to the low side of the load.
Use a PMOS FET on the high side and invert the gate-drive logic.
Add a charge pump circuit so the MOSFET gate can be driven above VBAT.