Examples:
We take best possible, scleable technolgy into play
We are conglomoration of companies serving various Industries with common theme.
[First name] envisioned use of technology is businesses to compute and improve efficiency.

Note: his potential customers are in the US.

Is he aware that his English is not the best? If so, he'd probably be relieved to have help with it. I've found that's usually the case. Many people realize that they're not good writers and are happy when someone else offers to deal with the problem.

If not, that's kind of hard... I'm not sure how you'd break it to a person either. It could be a problem if you went into business together and he insisted on doing more writing of his own. It could even be possible that there's no nice way to make him understand. Kind of like people who won't admit that they have bad breath, or that they shouldn't mix different plaids.

I've just always been straightforward. "There are problems with the English on the site." Some people are like, "yeah, of course — that's needs to be fixed!" and some are like, "What? No, we got a good person to do that." I just tell them that it wouldn't sound good/competent to native English speakers, and leave it at that. I don't think anyone has ended up angry at me about this, or thought I was being condescending, but I'm also very clear that my Greek sucks donkey balls... so.

To me, it's like the whole "do you tell someone when they have spinach between their teeth?" thing. I always will, because I'd want someone to tell me.

the spinach between teeth analogy gets to heart of the issue i.e. it's really dependent on where their mind is at regarding you.. hard to know how they'd take it. Obviously usually nobody really explodes with resentment but you're trying to also not upset them internally.

Try being objective / passive rather than using active or specific language i.e. "by the way there's a fair amount of copy editing the site could use if you get around to working on the text. feel free to drop me a note if you need me to check any specific passages. good luck!"

I'd start with a compliment, and then transition into the critique in a friendly, helpful way:

"Your site looks really great and I can see that you've put a lot of hard work into it. One thing I noticed, though, is that some of the text doesn't read as smoothly as a native English speaker would expect it to. I'd be happy to help you make a few tweaks, if you'd like."

Ask him if he would like help translating his website into "American English."