I'm sorry to hear that your horse is having such significant issues. With what you are describing, the two causes that immediately pop into my mind are Choke and Gastric reflux.
With Choke, if they are going to resolve on their own I usually see it occur within the first hour after the initial choke. If it doesn't resolve within that period of time, the muscles of the esophagus generally fatigue and can't push the feed bolus towards the stomach. The first hour or so after a choke is generally when you see the most nasal discharge and oral discharge, and may even contain some feed material. Merlin may have even coughed during this time. The longer it goes on, the discharge can look more like saliva with little feed material present. Initially I will place the horse in a quite area, and take away all food. I do leave water in there in case it clears so I can watch them drink, and see them swallow. You can palpate the esophagus along the left side of the neck. You can start at the top near the jaw, and run your hand down the jugular groove on their neck, and sometimes you can palpate the area of choke if it's in the neck region. Sometimes the choke is further down as the esophagus enters the chest and can obstruct at the base of the heart, or at the opening of the stomach, in which case you can't see of feel the area of choke.
With an uncleared choke Merlin can be susceptible to aspiration pneumonia
from aspirating saliva/feed material into the lungs since the opening of the esophogus sits above the trachea.
Generally in these cases your veterinarian will have to pass a nasogastric tube and try and push and lavage the obstruction into the stomach. They will also likely place Merlin on antibiotics to treat/prevent aspiration pneumonia (I always place the horses I see with choke on an antibiotic). The longer it goes on, the higher the likely hood of aspiration pneumonia or damage to the esophagus and the more difficult it is to treat.
Once it's cleared, I don't typically recommend feeding hay
for a good 24 hours or so, and when I do, i recommend soaking nit or wetting it down to soften it up a little. Usually I will start horses back on a Mash usually using wheat bran as wet as they will eat it to stimulate GI motility, but also be very easy on the esophagus. I'll also have them on bute or banamine for 3-4 days as well to reduce inflammation of the esophagus.
The other though of Gastric reflux can be even more severe as it is a back-up of fluid in to the stomach. This can be a blockage, or proximal illeus (shut-down of the small intestine). Generally with this they are showing signs of significant colic and pain. But it can lead to nasal and oral discharge. I would suspect choke over this but having your veterinarian pass a nasogastric tube is the way to diagnose this as well.
I would highly, highly recommend having Merlin seen by his veterinarian and get this treated as soon as possible to minimize any potential complications.
I hope this has fully answered your question. If you have any further questions, please do not hesitate to ask. Also, please remember to rate my response in the justanswer rating system as it helps me continually improve the my responses.