Greetings, I am Dr. Pat. I have worked with birds exclusively for many years. I have a parrotlet myself.
This is an emergency, and you should get him in to a bird-experienced vet first thing tomorrow.
Is there any bruising on his head? How is his breathing? Can he stand, is he coordinated?
If you feel comfortable with it, examine the bird thoroughly, using gentle restraint via washcloth or hand towel: do not restrict the chest or hold around the body. Check the eyes, nostrils, mouth and beak if possible, having a good look in there for mucus, redness, masses or anything else unusual. Palpate the tummy for pain, fluid, lumps or anything else (eggs, if female or unknown). Check all the joints for swelling, pain, and mobility. The feathers should be parted to view the skin, muscles and skeleton below; this can be done using a q-tip with isopropyl alcohol or KY gel. Look for bruising, lacerations, injured feathers.
Your job is to keep the bird warm, safe, quiet, and confined; and to provide adequate hydration and calories.
Move the bird to a box or carrier with soft towels in the bottom, no perch, and food and water in low bowls that can be reached easily. Put the whole thing on a heating pad on low or medium. Check it frequently, no overheating allowed! Keep the unit partially covered, warm and quiet. White paper towels or white cloth towels will show the true color of the droppings. Small animal/reptile boxes are great for this purpose.
The bird, bowls and unit must be kept very clean.
Here are some helpful links:
Do not try to force food or water. Pedialyte or electrolyte replacer can help but many birds do not like them; when in doubt, plain warm water is best. They can hydrate from oral fluids almost as quickly as IV if the GI is functioning properly. You can offer warm cooked rice, pancakes, cornbread, grapes, melon, greens in addition to normal food.
Pet/feed store medications and home remedies are harmful, ineffective, immuno-suppressive, and make them much worse and may interfere with the veterinarian's diagnosis and treatment. Do not use them. Homeopathy and natureopathic techniques do not work in avians and can actually be very dangerous.
I know it is expensive, but you may not have many home options, because the first thing you need a vet for is to find out what is going on. Treatment is only as good as the diagnosis. If you call around, you may find a vet to work within your means.
I really must stress that you need a bird-experienced person, and not just a vet who advertises that they care for birds.
You need to take your bird to see an avian-experienced veterinarian ASAP for complete examination, diagnosis and appropriate treatment. Check
https://aav.site-ym.com/?page=basiccare click on "find a vet"
for members of AAV in your area or call your regular vet and see who they recommend; ask if they really have worked with birds a lot.