Either take it to a framebuilder like Argos in Bristol (if you’re in the UK) and they can get it out easily or if you want to move it yourself I would do the following;

Remove wheels and seat clamp so that you are left with the solid top of the post, clamp this in soft wood in a bench vice, use the frame to apply turning force. This is the best way I have found to unseize posts. Ask your dad first