As far as I'm aware, they have mostly stopped issuing PU letters as it's technically possible to go back to China with the old z visa route now. They just don't seem to actually be doing it!
Everything the Chinese government has released since they first started issuing PU letters last year has indicated that foreigners must be in their home country to get their documents processed. I've seen probably a dozen of these documents. I also know dozens of people who left in 2020 and still can't get back in. Business visas are an option, but if he plans to teach there, he'll be working illegally. Which many do of course, but I wouldn't do it
that is really impossible right now. Bringing him to Thailand won't help you get him into China. The Chinese will only issue work visas in the home country of the applicant.
Right now, you can convert visas in Thailand without leaving. That could change at any time though... Although with the recent negative change in the Covid situation, I would expect that to be ongoing for some time
I would also advise to find one that doesn't insist on prepaying everything. Never know what can go wrong these days. I only paid 5k before arrival in my ALQ in Pattaya