He has just had lunch.
Il vient de déjeuner. — He has just had lunch. Then how do you say "He is coming for lunch"
"Il vient dejeuner" and "Il va venir dejeuner"
and some examples:
"Il vient dejeuner demain"
"Il va dejeuner avec nous aujourd'hui"
(In the last example, I left out the word 'venir' because the sentence makes it clear he is having lunch with us, so clearly he is coming, and 'vient' is superfluous).