Usage of lean/sit

[Daemon99]

Can some one please help me with these questions?

My friend is sitting back in my chair. (Can I say sit back?)

Do the following sentences sound correct? Are there any better ways of putting it?

1) His back is resting on the backrest
2) His back is leaning on the backrest
3) He is leaning on the backrest

And there is a towel lying(any other word?) in between his back and the chair's backrest.

I want to ask him to move forward so I can take my towel. Can I say,

1) Lean forward
2) Move forward

Any more suggestions are welcome.
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Non-native speaker of English
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I intend to live forever - so far, so good.

[Mister Micawber]

.
Use these:

3a) He is leaning back.

1) Lean forward.
.
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Native English teacher at Mister Micawber's