Alright. I'm not an expert, but I've got a couple corn snakes and did run into an issue with regurgitation last year.
First thing first, you need to increase your hot side temperature. I'm more familiar with farenheit, but converting to Celsius, your hot spot needs to be 29.5, not 26.5. Try and measure it with either a probe thermometer or a temp gun (cost about $10-15 at walmart) below the substrate above the heat mat (or on the surface if you are using heat bulbs)
It is most likely that your corn regurgitated due to not enough heat to digest his meal. What happens if the heat is too low is the food starts rotting inside him before he can digest. They heavily rely on the heat we provide to digest. Please don't be too hard on yourself, but do learn from it.
I believe there is a sticky thread on here somewhere about regurgitation and protocols for afterwards.
I'll give you a basic rundown of what to do, but please go look for that post to get more detail from someone with years and years of experience.
Due to the loss of stomach bacteria and the burns it causes up the throat when they regurge, you will need to not feed him for 2 weeks, a full 14 days, to give his throat time to heal.
Get some of the Nutribac vitamin powder. You will want to use this to help give him back beneficial bacteria when you do go and feed him. This is just dusted on a slightly damp feeder.
Feed smaller prey for the next several meals (each at a 1 week interval to allow him to build back up his gut flora. At pinky size typically feeding is every 5 days, but this is an exception).
Since your snake is eating pinkies, this means you will need to cut the pinky in half and offer him only the head. (I would suggest you do not freeze and use the other half. Refreezing is generally not good for the feeder and will probably be a mess when thawed again.)
Do this for at least 3 consecutive meals where there is no repeat regurgitation.
Can you give more information on your enclosure and snake such as his weight?
His weight is something you will need to watch closely at his young age (and presumably size). Even if the usual recommendation is to not handle at all until he has eaten for you, a regurge changes things. You will want to handle him for a just a minute once a week to get a current weight and ensure he isnt losing weight while you wait for his throat to heal up. Be calm in handling and as long as it is brief he should have no ill effects.
If he loses weight, I'm not sure what the ideal method would be, but you will probably want to bring him to an exotics vet and explain this to them for professional advice. If his weight doesnt go down more than 10% you are likely just fine with the above method.
As for the substrate, what is it? How large is the piece? (Proportional to his head, perhaps).
In many cases it won't be an issue if he does swallow and he may be too tired to spit it out after the regurge. I would he surprised if it is still there in the morning.
He probably wiped his face after regurging and it got stuck to him due to the stomach acid he spit up with the mouse, and it will probably get rubbed off or spit out just fine.
Sent from my SM-G950U using Tapatalk