Dual Citicnship for Thais in Norway

[Big Boy]

So if they take UK citizenship at age 18 their not required to do service in Thailand.

Seems a fair assumption - military service is for Thai nationals, if you've taken a foreign citizenship how can it apply?

I think the issue comes with dual nationality like my son. Also in the OP, especially when Norway also has National service. The dual national will be liable for both.

If Matlot's boys drop their Thai nationality, then they will no longer be liable.