"Later" can be either an adjective or an adverb. When it is used as an adverb, you can use "later on" instead. For example, "Let's talk later" = "Let's talk later on". However, when "later" functions as an adjective you cannot do this. For example, you can say "In the later hours of the evening, I get tired". But it makes no sense if you replace "later" with "later on".