Q:

I didn't think him to be so honest

I didn't think him to be so honest.

How can we change this sentence into comparative degree without changing its meaning or sense?

A:

I didn't think him so honest. 
To be more clear, you can rephrase this sentence as follows, without any difference in meaing.
I thought him not so honest.
To make it a omparative sentence, you can say "He was more honest than I thought", as commented by Jim.

