Hi
Please take a deep breath. It is highly likely that nothing is going to happen; that is my own occasional experience with jammed syringes. Just keep a casual eye on him for a day but if he is eating on his own straight afterwards then he is not having breathing any issues. The juddering was just from him swallowing it all. In order for an aspiration to happen, he must have been taking a breath just when the syringe unloaded.
Unlike newborn babies for which aspiration is usually fatal, adults can hold far more food in their mouths and even more importantly, they have got an immune system which babies haven't yet. It is the lack of the latter that is fatal in babies.
Most of anything going down the wrong way is dealt with the same way as in humans: it is got out again with reflexive coughing.
PS: Rather than making the feed more watery and actually getting less solids into him than usual, it is easier to feed the usual mix but offer him water, as much as he is willing to drink from a syringe (instead of having it it squirted into him), in between feeds. That allows you to keep track of how much crucial solids you are actually getting into his gut.
Please keep in mind that while piggies will drink a bit more, especially in high humidity, they do not sweat or pant but shed excessive heat by increasing the blood flow through especially the ears and paws. They are not losing as much water as humans or dogs. If they feel damp, then it is from condensation in too tight, hot housing.
Concentrate on keeping the piggy room as cool as possible (and not just the cage) and keep the water in the bottle cool and refreshed. Also keep in mind that the need to drink comes before the need to eat. A piggy that is eating on their own at least partially is already drinking on their own.
The temptation to overfeed watery veg and fruit in hot weather!
Hot Weather Management, Heat Strokes and Fly Strike (See chapter 1)