"I think he'll [be able to] [figure/work] it out [by himself]" is probably fine for this.

Adding "be able to" make it sound a little more polite, at least to me as a British English speaker.

I don't see much difference between "figure" and "work", although "figure" feels like a more educated way of speaking.

"By himself" is implied, but you can add it if you like. Some people prefer to speak like that, and it does carry a stronger tone of "stay in the car, you don't need to tell him".

As an aside, your direct translation is also acceptable with some small modifications: add "I think" at the beginning, change "will" to "can", change "it" to "that", and remove "now":

I think he can understand that by himself.

"I think he'll [be able to] [figure/work] it out [by himself]" is probably fine for this.

Adding "be able to" makes it sound a little more polite, at least to me as a British English speaker.

I don't see much difference between "figure" and "work", although "figure" feels like a more educated way of speaking.

"By himself" is implied, but you can add it if you like. Some people prefer to speak like that, and it does carry a stronger tone of "stay in the car, you don't need to tell him".

As an aside, your direct translation is also acceptable with some small modifications: add "I think" at the beginning, change "will" to "can", change "it" to "that", and remove "now":

I think he can understand that by himself.