Me and my Dad have had another look at the config file.
Firstly, we were originally editing /opt/retropie/emulators/retroarch/retroarch.cfg however it seems like we were editing the wrong directory…
This is the correct directory:
/opt/retropie/configs/all/retroarch.cfg
These are the lines of code that we used:
# Saves state.
input_enable_hotkey = q
input_save_state = x
# Loads state.
input_load_state = z
My Super Pi Boy project is now complete!
Also, make sure that you uncomment the input_load_state = z line.

Thank you so very much Floob for helping me with this issue. I hope that this post will help everyone who needs to overcome this problem.

Thanks again,
Josh