Can my son get a long-term visa to work as a dive instructor in Thailand?

His employer in Thailand can sort out his Visa.

Contact your local Thai embassy would be the first step 🤷‍♂️

If they are suggesting he just comes over on a tourist visa and 'work it out once here' forget it

He needs a non B visa and a work permit. Visa comes first. They should provide him with all required paperwork to apply at the Thai embassy*before* he travels, as he won't be able to work legally without it. He *could* just travel on over and then sort it, by going to a neighboring country for the visa at a Thai embassy, but he would not legally be allowed to start working on a tourist visa or visa exempt entry. If the employer wants him to start sooner rather than later, or if he depends on getting paid from the get go, it's better to have the visa before coming here. In tandem with the visa they also need to provide required paperwork for a work permit. If they are not willing or able to provide all the required documents for these two things, run away.. Not worth it to work illegally and risk deportation/fines/jail time.

Yep company needs to supply paperwork gor a non-b outside the country and then for a work permit and then for a visa extension in the country. If they don't, walk away.