Will he be looked upon with suspicion at the border control?
Probably not only because of not having used his visa yet, but it's possible.
[I]s there a set of documents that he should be carrying just in case the border immigration officer may ask?
If he's staying with you, he should have your name, address, and phone number, and if you're not meeting him at the airport he should have a way to get to you. Note that he'll need these whether or not the officer asks.
I am asking him to carry some USD in cash (or a forex card), an employment verification letter, a utility bill from back home.
These are not likely to be needed. He should avoid presenting these documents unless the officer starts asking about his ties to his place of residence. Presenting these documents could arouse suspicion that he is overly prepared, which could imply some sort of improper activity.
Is there a reason for me to worry?
No. The likeliest problem seems to me that you will make him so nervous about passing through immigration controls that he will find himself in secondary inspection because of his nervousness.
Back then I had filled in form I-134 for him, which probably he did not even end up using. Should I ask him to carry this form as well?
No. If he did use it, it will be on record. If he did not use it, then he got his visa without it and there's no reason to think that it would be necessary to gain admission at the border.
You should be available by telephone when he arrives so that, if officers become suspicious of him, they can call you to confirm his story.