When droppings collect at the vent there could be a few things contributing to it, perhaps one of the most common being an egg needing to pass. Quite a few owners are shocked after many years believing they have a male bird and finding that not only is it a girl, but it’s in trouble. When an egg is stuck (egg binding) it’s an emergency situation that needs veterinary intervention quickly.
Giving the bird a soaking in a shallow pan of warm water will remove the droppings and if there’s an egg bound, may relax the muscles enough to allow safe passage. Either way, male or female, the warm, shallow bath is applicable.
*** I know this is very likely a male since you've had him long enough to hear his vocalizations, but it would be irresponsible of me to not post this 'just in case' ***
Another possible cause of vent pasting would be urate crystallizations which are not uncommon in birds on a largely seed diet.
Further causes would be impaction from eating grit or other non-food items.
While these are the most common, there are too many other possibilities to mention including bacterial or fungal infections or any number of diseases, even internal tumors or other growths you can't see or feel.
In any case, it’s necessary to have a hands on vet evaluation done to get to the bottom of the issue.
If he stops eating, acts lethargic, looses his balance shows tail bobbing (the tail flicks up and down throughout the day) it could be an 'urgent care' situation and you must not hesitate to engage a professional for a hands on exam.
What you can do while securing a vet to see your companion is supportive care.
Most birds will need a heat source to maintain body heat while you’re getting their medical treatment lined up or while you’re on your way to see the vet with them.
If perching is a problem, lower the perch to just a couple inches above the floor of the cage so falling doesn’t injure them. You might also want to remove the perch completely and layer newspapers on the bottom or put a tee shirt there. We use T-shirts because bath towels tend to have looped surfaces that can entangle little toenails.
Putting a heat source into the cage may be necessary since a sick or distressed bird may lose body heat. I prefer a non electric source and use rice socks.
Use a thick, clean sock and fill it ¾ with plain, raw white rice. Knot the end and microwave it for about 1 ½ minutes. Shake it afterwards to distribute the heat and be sure it’s not too hot.
You can layer a thick towel on one side of the cage, secure with clothespins out of the reach of the bird’s beak – then clip a heating pad over the towel and set on low. Check often to be sure it’s not overheating and that the bird isn’t gnawing through. A side attachment like this will allow the bird to move closer or away as needed.
If the bird is bottom-bound, install a heating pad under just a portion of the base, again, set on low. Check constantly. These things have a tendency to turn off or overheat. I prefer the rice sock option as far safer.
I think you'll be able to get him back on track soon enough for the longer daylight hours to turn his beautiful song on again.
Please let me know how you make out ok?