If he's breathing heavy and twitching his head a lot, he could have the beginnings of a respiratory disease. I'd give him some electrolytes and probiotics, and isolate him from the other birds (if you haven't already). Keep him warm and out of drafts. I recommend also getting some antibiotics in case it is a bacterial respiratory disease instead of a virus. Tylan50 is one of the best antibiotics for respiratory diseases. It is usually given as an injection, though it can be given orally or in the water (if using a special powdered form--this is more difficult to find). You'll need some small (20-22gauge) needles and some small syringes to give it as an injection. Give Tylan50 once a day, injected into the breast muscle, for five days. The dosage is 1cc for large fowl, .5ccs for bantams. Here is a link to giving chickens injections:
http://shilala.homestead.com/injection.html
There are other things that could be wrong with him as well, but so far the symptoms aren't very specific. Many chickens diseases cause chickens to be lethargic and breathe heavily. Update us if he begins to show any more symptoms, and if you take him to the vet.
You need to get him to eat enough. He is likely producing liquidy droppings because he is sick, and because he isn't eating enough. Tempt him into eating by offering some scrambled egg, yogurt, or applesauce. Try some moistend chicken feed, too. If he doesn't eat solid foods, give him some egg yolk (raw), and drip water and electrolytes into his beak. You can also try mixing feed with water until it is a soupy mixture and putting that into a plastic spoon. Hold it up to his beak (you'll not be able to do this very well if he's not a tame bird) and tilt the spoon a little while pressing upwards against the beak. This will sometimes cause him to swallow and open his beak a little. If he doesn't eat anything even after trying giving him food by eyedropper or by spoon, think about tube feeding. I've never done it personally, but here are some helpful links:
https://www.backyardchickens.com/t/722041/how-to-t-feed-a-sick-chicken-and-give-subcutaneous-fluid
https://www.backyardchickens.com/t/...h-my-bird-pics-for-visuals-very-detailed-post
Hope this helps, and please keep us updated!
