Here's some info that will make it clearer https://www.thaicitizenship.com/thai-military-service/. I believe if you register his birth with the Thai authorities, then he will be on a house book (Tabien Baan) somewhere (I think, but not 100%), therefore eligible to be drafted. So, it mainly depends on your future intentions, whether you plan to move to live in Thailand or not. If you never have plans to move there, then perhaps it's a choice your son can make when he gets older as it is he who has to do the time. As others have said, there might not be a draft at that time, so you could register if there's news at that time. However, if you plan on relocating to Thailand, then it's better to register him, as the easiest solution to avoid the draft is the Territorial army induction completed in his school years (note, you need to enroll him in a school that offers this option). My son is doing his now and it's only a short time each year. If on the other hand, you only plan to visit on occasion, then he would only have the citizenship he entered Thailand on and the visa implications along with it. There could be a scenerio that if the draft was still ongoing, but your son was in university elsewhere, he would have to run the hoops to defer military, and if he doesn't want to do it, then would have to remain outside Thailand until 30yrs old. It's a tough choice, so I would choose wisely depending on your future plans for you and your family. Bearing in mind, his connection to Thailand is an important one, so as others have said, for the hassle of doing it later may be more of an issue than not doing it now.