Il ne croit pas en grand-chose

Hi Zhe,

The verb is

croire en quelque chose (to believe in something)

Il ne croit pas en Dieu= He doesn’t believe in God

Il ne croit pas en grand-chose=He doesn't believe in much at all

Hope this helps!

Laurel, 

No, you cannot drop the ‘ pas ‘ from the negation ‘ (ne) pas grand-chose ‘ , just as you could not drop the ‘ plus ‘ from ‘ (ne) plus grand-chose ‘.  

                “ From the lesson: You can never use grand-chose to express much, this exists         solely as a negative expression. “

In the verb form ‘ croire en… ‘, en is a preposition that impacts the meaning of the expression. It is not the adverbial pronoun ‘ en ‘. 

There is also the form ‘ croire à ‘ - again the preposition ‘ à ‘ impacts the meaning of ‘ croire ‘.

See links to Laura Lawless site.  

There are 2 ‘fixed’ constructions  in the phrase “ il ne croit pas en grand-chose “ 

1. Croire en qqc - to believe in something. The verb form here is ‘ croire en ‘ 

and it is ‘negated’ by

2. (Ne) .. pas grand-chose - not much

 https://www.lawlessfrench.com/grammar/croire-lesson/

 https://www.lawlessfrench.com/expressions/croire-expressions/

The 'en' in the question is the preposition 'in' - «He doesn't believe in much». This is different to what you wrote, which would be "He doesn't believe in it much" (context would be needed to know what 'it' was).