You are talking about the stainless bolt that passes through the aluminum steering quadrant to the Stainless rudder post? I suspect you are the victim of galvanic corrosion. You could try heating and cooling the bolt to try and break the bolt loose - make sure you are using a 6 point socket and not too much force as breaking the bolt is very bad news. (for heat use either an acetylene or mapp torch - heat the bolt and watch the temp as aluminum melts at around 1200ºF) Some suggest a variety of chemical penetrants or bees/candle wax. Tap the head of the bolt with a hammer occasionally. Slow, steady, gentle and lots of patience unless you want to end up drilling out the stainless bolt.
Something else you could try - Take the boat on the trailer to welder who deals with stainless and have them repair the weld/crack with the rudder still on the boat.