Do you know when he will be back?

[keannu]

Is it possible to say "Do you know when he is back(or when he comes back)? instead of the following?
My grammar book says that future tense can't be replaced by present tense in noun clauses.

gz13)Do you know when he will be back?

 

[5jj]

Do you know when he will be back?

If your grammar book says that, it is wrong. That sentence is fine.

 

[keannu]

You mean these two are fine when referring to the future?

1. Do you know when he is back?
2. Do you know when he comes back?

 

[5jj]

You mean these two are fine when referring to the future?

1. Do you know when he is back?
2. Do you know when he comes back?

I didn't say that, but they are possible.