If you give a specific time when using "till" (or "until") then yes, that would be the point at which you are saying something ends, for example:
I work till 5:30pm.
This would mean at 5:30pm you stop work.
As "late night" isn't a specific time, the meaning is different.
He works till late.
Idiomatically, this means it is already late when he stops working. As there are gradations of what is "late", this isn't really a cut-off point. It could have been late for hours when they stop. Also note that, from the perspective of a day's activities coming to an end and going to sleep, the early hours of the next morning would be considered "late" - for example, 1am would be a "late" bedtime, so "late" really has no end.
This wouldn't mean the same as "he works late at night", which could mean that someone begins work late at night.
If, for example, you said "he works till early evening" that is a little more specific. There might be a time range that people consider to be early evening but it at least suggests that work would stop before "late evening" or "night".
"Late at night" idiomatically means during the late hours of the night. The use of the adverb is correct. "Late night" is used as a compound adjective to describe things that happen late at night, for example a "late night movie".