There was I, and there was him.

While the line is poetic, something with a similar rhythm is, "There was I, and he was there." It's a tad bit more natural.

My camera haunged out of from my neck and I picked a book from the cabin for some photos ideas.

He, my model, followed me.

I felt aslike a child, playing with my camera and finding some angles and places to portray him.

The content didn't make sense and; it was so absurd that we couldn't help but laugh.

For simplicity and to reduce your usage of conjunctions, you can use a semi-colon.
You could add "didn't make much sense" or "make any sense" for emphasis if you'd like.

There was I, and there was him.

Although not grammatically correct, it's poetic enough to let it slide

My camera haunged out of from my neck ands I picked a book from the cabin for some photos ideas.

He, my model, had followed me.

I felt aslike a child, playing with my camera and finding somein order to find the right angles and places to portray him.

I told him to stay outside the cabin while I took him some photos of him below the cabin.

This experience iwas simple, yet so lovely.

A lovely mMemory

For titles in English, the first letters of each word are usually capitalized except for some words like of, and, etc.

There was I, and there was hime were.

You can simplify it this way by mentioning you and the other person being in the same spot.

My camera hanged outdown of my neck and I pickedchose a book from the cabin for some photos ideas.

Usually, a camera can hang (down) from your neck, not out.
Saying "I chose" a book would be the better option over "I picked" a book.

He, mMy model, followed me.

Since you're already talking about your model, you don't have to mention the "he" at the beginning.

I felt aAs a child, I felt like playing with my camera and finding some angles and places to portray him.

Since you're talking about the past, you can mention when you were a child first, then say what you were doing.

I told him to stay outside the cabin while I took him some photos of him below the cabin.

I assume you're taking photos of your model, so saying "I took him some photos" would indicate that you gave him photos.
In this context, it would be best to word it as "I took some photos of him" as in you are taking photos of him.

I looked at the book I gave him, and he opened a random page and began reading.

This sentence has an unnecessary pause, it would be more fluid to add a comma to combine this sentence and the one below.

He opened a random page and startedbegan reading.

Using the word "began" in this context would be a better word to use rather than "starting" as in starting something up.

The content didn't make sense and it was so absurd, that we couldn't help but laugh.

Since you're making a pause after the word "absurd", a comma would go after it. Otherwise, it would be a run on sentence.

Thise/That experience iwas simple, yet so lovely.

I'm assuming this experience happened in the past, considering your next sentence below mentions "whenever you think of it" meaning it has happened before. So, you wouldn't say "this" experience as that indicates the present, it would be better to say the, or that experience.

OPTION A: There was I, and there was himnext to him. OPTION B: There he was, next me. OPTION C: There we were, next to each other.

Many other options exist.

My camera hdangled out offrom my neck and I picked a book from the cabin for some photos ideaswhile picking up a book with some photographic ideas from my cabin.

What kind of "cabin" were you in? (A train cabin? A cabin on a yacht? A log cabin? More detail needed.)

The content didn't make any sense and it was so absurd that we couldn't help but laughing.

This experience iswas so simple, yet so lovely.

There was I, and there was hime were, he and I.

Your writing seems to be written with a poetic tone, therefore what you wrote could still be used to emphasis the individuality of you both. However, even with a poetic tone rather than "him", "he" sounds better, but you have a bit of artistic freedom here.

I felt aslike a child, playing with my camera and finding some angles and places to portray him.

I told him to stay outside the cabin while I took him some photos of him below the cabin.

My camera haunged out of on my neck and I picked a book from the cabin for some photos ideas.

For past tense of hang, use hanged only when referring to someone being hanged to death.
Also, consider this alternative for style and easy-to-read-ness:
"With my camera on my neck, I ..."

I felt aslike a child, playing with my camera and finding some angles and places to portray him.

"as a child" could mean "in the role of a child".

I told him to stay outside of the cabin while I took him some photos of him below the cabin.

I'm not 100% sure what you want to say here. You took photos of him from inside the cabin maybe?

He opened to a random page and started reading.

More natural alternative:
He turned to a random page

The contenxt didn't make any sense and it was so absurd that we couldn't help but laugh.

I can't tell you why but using the word content here feels wrong 😅