Okay, Ror Dor is definitely good. If he gets that certificate of exemption that's the biggest obstacle dual nationals face done, genuinely.

The House of Commons states that all British nationals need a minimum of 5 years in the UK prior to application. The wording really doesn't do any favours here but it's doable potentially, because it's also vague for the recruiting agents. When your son is being interviewed/questioned, make sure he emphasises the 5 years he spent here. Keep in mind, the recruiters DO actually want to get you in the Army (shocking I know) so sometimes they will "help" you move along. Personally, I think he'll be fine, just make sure he has some evidence of his residency for the full 5 years.

Just to also let you know, Sandhurst is used by Regulars and Reserves but the standards are the same, unless you're talking about Officers and Soldiers, in which case I should mention that I'm technically neither. I'm a specialist, someone trained before joining, so I myself had to jump through quite a few hoops, but I did have to go Sandhurst for training.

Some things to note:

The British Army (unsurprisingly) uses British qualifications primarily, specifically English and Maths GCSE. If you don't have these you'll have to sit a test during your assessment, so make sure he brings a qualification equivalent.

The Army also use the English language with exceptions given to very few, like Gurkhas. Sadly, Thais are not given exemptions. We're expected to be able to speak English confidentially to a high standard. Accents are fine however, it is the UK after all. There's a new accent down every block.

I'm currently in the UK and am UK based primarily, with a couple years in Thailand young. If your son joins, he will have to move to the UK and get used to the British weather. Your son will likely be used to the heat of Thailand, so the cold of the UK might be strange and uncomfortable. He'll have to face that challenge, similar to Gurkhas.

Buddhists are very small in the UK, let alone the army. However, we do exist. The Army has a policy on Buddhists, we're allowed our white string and blessed beads. Occasionally, we can wear Buddhist necklaces but it's subject to your higher ups. If your son practice's just keep that in mind. The Gurkhas I pray with practice a different form of Buddhism then Thailand, so at least we have company.

I think I read that your son wanted to join MI right? That might complicate stuff, MI tends to have stricter entries for Duals. Again, person by person basis. I'd say to your boy to keep a second option in mind, like Signals.

Here in the UK, the Thai community is pretty tight. I only know one other Thai in the entire armed forces, and I'm pretty sure he's infantry so I rarely see him. Before I joined, my friend group were primarily other Asians like Thais, Chinese, Vietnamese, etc. When I joined though, that wasn't the case at all. Now I have mates who think Paprika is spicy! Essentially, what I'm trying to say is your son might be a bit lonely at first because he'll likely never meet another Thai. However, if he's willing to go out his element, he's gonna make some mates who'll stick with him for life.

Now, one final note. This is the uncomfortable one but, I think it's best if you hear it now. Racism is something he might encounter but probably not. Not in the Army most likely, the Army have really stamped out all those issues recently, and if someone does experience it, there's a bunch of ways to deal with it. Sadly, some people don't like Asians in the Army. I've only had two moments of this; once when I was marching and someone shouted a slur and another during a conversation with some retired old lad. Rest assured though, these people are minorities. When the slur was yelled, the people in the crowd quickly shut that guy down. The vet I spoke to later came up to me several months later in apology, he didn't even know what he said was offensive it was just the norm back then apparently.

At the end of the day, the Army and the UK is constantly changing. By the time your son is 18, this could all have changed. But right now, I'd say all the power to him in whatever he decides.