Je vais l'appeler.
Here's a breakdown:
* Je - I
* vais - will (from the verb "aller" - to go)
* l' - him (contraction of "le" and "le" - the + him)
* appeler - to call
You can also use the future tense:
Je l'appellerai.
This translates more literally as "I will call him," but the first option is more natural in everyday French.