My partner has a son (now aged 31) who holds a UK passport and a Thai passport. He is currently living in Thailand on his UK passport making reports to immigration every 90 days. He believes that he is unable to use his Thai passport as he has not complied with his military service obligations (he failed to appear for the military service lottery when asked to attend, while a student in Thailand using his Thai passport, aged 21).

Does his military servicve obligation expire? If so, when? If not, is there any way to regularise the situation w,ithout undertaking military service?

I would particularly welcome thoughts from anyone Thai with direct experience of mililary service.

might be wrong but I'm sure I read somewhere that it expired at thirty.