The other option is a 'walk in the park' -so they say, anyway- ROTC type service for 6-8 weeks for college students. Not sure what the formal procedure is for Thais who are overseas into their 20's... but for myself, no draft letter ever came, and no one has ever said anything when I have renewed my Thai passport and id card. I'm 32 now, maybe some flag will pop up when I renew again around age 40.
edit: renouncing one's citizenship is definitely a burning one's bridges type of move... possibly sentencing the kid to a life of visa running without property ownership should he/she decide to live here one day. And one in fact never knows... even I never thought I'd spend a good portion of my life in the LOS.