deedles - Is Liam just scared of the big potty that flushes or all potties in general? DS went through a brief period of being afraid to sit on the big potty. I just bought a portable potty that doubles as a stool for him. Now we only use it as a stool for when he uses the big potty, and for when we go camping and the toilets are too far away.
Good luck - you (and he) are doing great with this big change!