M.B.
Congratulations on breastfeeding for this long! Your son is so much healthier for it! I have never tried it, but I've heard that using cold cabbage leaves on your breasts helps with engorgement. Just roll the leaves under a rolling pin or cut the tops of the veins so that they will bend, and use the leaves to cover your breast. You can put a cold compress on top of the leaves and then put your bra on to hold everything there, and change the leaves when they wilt. Some people think that this diminishes the milk supply, so since you are weaning, you could leave them on all day if you want.
I would also pump when you feel really full, but only to the point that it's not uncomfortable anymore, not to completely empty. The idea being that you are tricking your body into making less milk since you aren't using it all. You could even save this extra milk to make your son's cereal with! Best of luck!