Your body knows that the pump is not your baby and sometimes it just decides that it's not going to cooperate. So you may be at the point when your son's consumption has dropped enough that you're not producing as much milk plus your body has adapted to the pump and has decided to not bother providing milk for it! Also, milk production is very in tune with what you are feeling and thinking. I know it's difficult, but try not to stress about the drop in milk and just relax and think about your handsome little man.
However, it could also be that your pump or some of the parts has worn out. Does it feel like it's pumping with the same strength? Have you ever been able to manually express milk, and if so, can you still do so?
If you don't need the milk to feed him then you may just have to drop the pumping session. Your body will likely continue to make an appropriate amount of milk at the times he nurses, so there's absolutely no need to worry about weaning at this point if you stop pumping! And if you do stop and feel that your milk supply is dropping then adding back in a pumping session should help bump things back up.
If you do need the milk for him, then you could consider pumping for about 10-15 minutes right after he nurses - you won't get too much milk at that time but it sends a powerful signal to your body that you need more milk.
Good luck, and congrats for continuing to breastfeed past one year and while you are working. You're awesome! :)