I would be far more worried, if you were NOT concerned, for your son's safety, - that's a parent's "JOB"! But, - don't worry, - as others, before me, have said, - he cannot join up, for at LEAST THREE YEARS, yet! And, even, if he DOES join up, - there are many months of TRAINING, to undergo, before he achieves his aim! For him to achieve officer status, though, will NOT be easy, at ALL! It means, that he will require far LONGER training, than non-commissioned soldiers. And, there IS, of course, the possibility, that, - although, HE "WANTS" Sandhurst, - "SANDHURST" may NOT "WANT" HIM! How would he cope with the disappointment IF that happened? From YOUR viewpoint, THAT is the "BEST CASE SCENARIO"! Would he be CONTENT, to join the "RANK AND FILE"? I don't KNOW him, - so, I cannot TELL!
In the END, though, - if he still wants to go, when he REACHES eighteen, - the fact is, that you can do NOTHING, to STOP HIM! I am SORRY, if that was NOT what you wanted to HEAR, - but, THEY are the FACTS!
May I wish you, - ALL, - good luck, for the future!