"À plus tard les garçons !"
I cannot hear the difference between singular and plural, as well. I hear this frase like: "a plu tard le garçon". How do I to differenciate it?
'Le' is pronounced like a short 'luh' sound and 'les' is pronounced sort of like 'lay'. That's the only way to hear the plural for a lot of nouns.
When you use "à" with reference to time, it can be translated "until." For example, "À demain" is the expression for "See you tomorrow." "À plus tard" means "until later." (see you later!)