But that said he should have no problem coming in and out on visa exempt stamps as long as he can prove he works OS and has income from work outside Thailand. He should carry pay slips and proof of job with him when he enters. I had an oil worker friend from Alaska who worked 30 days on/off and visited every second month for years with no problem.
There was a recent case on here where a Thai with dual passports showed both on arrival and the IO stamped the foreign passport for 30 days. She was going to have to leave and return to not be on overstay. Don't give them the chance to stuff it up.
Depends where you are. In Hua Hin they don't care. I just renewed my retirement extension and asked about updating my TM30 and she said if I'm still staying at the same place, no need to report. I'd only been back a few weeks.
The 4 diseases that need to be checked against are extremely rare or non existent in advanced western countries. My GP in Australia just signed it with no bloodwork. But I've been seeing him for many years. YMMV