The question the Immigration Officer is asking themself is "is this person a genuine tourist". They could come to the conclusion that if you are working in Thailand that it is less likely that your boyfriend is a genuine tourist.
Unfortunately we can't tell what the Immigration Officer is going to think. All you can do is ensure that he gives himself the best chance of entry by having if asked 20K Baht (or equivalent) in cash, proof of accommodation and onward ticket. It sounds from your answers to others he will be able to show these if asked.
If you had been asking the same question a month ago I would of recommend for him to apply for a tourist visa. This would of increased his chances. It is possibly a bit late to apply now.
after working at an airport I can say that nothing is done randomly in immigration. Everything is done on evidence. Why would you create work for yourself for nothing?
you can do very little at the British Embassy. To renew a British passport in Thailand you need to contact VFS. They have offices in Bangkok and Chaing Mai.
you are much less synical than me. I wish I was more like you but 36 years of visiting Pattaya has informed me of who are the good guys and who aren't.