Most likely needs a liittle grease and a gentle application of excessive force
My next attempt would be putting a wooden dowel/stick through the eye of the big vertical pin and rotating anti-clockwise and try to feel if it is just pivoting or if possible has threads instead of a pin. Does the boom move freely port to starboard? Probably some corrosion/galling going on from the combination of metals and a non-user-friendly design for servicing.
Also check for any brand names, part numbers, etc to ask about or search for online.
@Yellowtuip, that horizontal pin looks swaged both ends to me, I could easily be mistaken.