The reason they say you can't over feed a breast fed baby is because they will spit up any extra they take on.
However it is unusual for them to take on extra unless you have a forceful letdown that they can't do anything but swallow, or they are feeding to reduce discomfort (say caused by reflux) because a hormone triggered by both sucking and the fat content of LOs stomach tells them to stop sucking and go to sleep.
It may be that his comfort feeding is stimulating multiple letdowns that he then can't help but swallow, so spending the evenings in a reclined position with him sleeping on top of you, or resting against you (rather than being held slightly under the boob) will help him to avoid drinking letdowns that he doesn't want.
Also do you have a big oversupply - everyone has oversupply in the beginning but if yours is extreme it could be that he overfills his stomach before there is enough fat in there to cause the hormone to tell him to stop sucking. Milk from engorged boobs is much less fat rich than milk from "empty" breasts.
I've never heard of a baby choking on spit up - that must have been so scary! - but as you say, you woke up and I think you would again because that is what we are attuned to do. It is also why it is recommended that we are always in the same room as our sleeping infants, so as long as you know you will be near him I think you can feel confident that you'd be able to help him if he was struggling.