You would describe someone who tends to lie as a "liar." "Is lying" means that he is lying right this moment.
The following is not a universal rule, as some verbs have unique words to do this and some have different spellings (like "liar"), but if you want to describe someone who frequently does an action, in English we usually add "-er" to the end of the verb. To describe someone who fibs a lot, a (fib being synonym for "lying"), we would call them a "fibber." Someone who codes is a coder. This grammatical structure frequently describes an occupation (a job), but can be used for anyone who does something frequently.