Hmmm....little short on time and really don't want to get into an argument on this as happened last time.
The official position, and for those who wish to argue, the information is available online, is that your son cannot have dual citizenship after he turns 18 (could be 21, can't remember). At that time he should choose which citizenship he wants to retain and denounce the other. If he chooses Thai then he is liable (although its unlikely) for national service. Only a Thai female married to a foreigner can have dual citizenship.
That's the official position although there will have been many thousands of Thai/?? citizens who have lived in the UK for many years and kept both passports.
Personally I'd rather make the choice. You never know what's been given the blind eye in past years but now enforced by the military government. Who's to say he wouldn't be arrested if he visits Thailand using his Thai passport or tries to renew it?
Of course, making the choice throws up other issues.....................if he chooses to be a Thai citizen then surely he would need a settlement visa to return to the UK?
I know none of these problems have ever come up for many thousands of others - I'm just telling you the official line.