"Public" DNS servers are public in the sense that they are available for the public to use by the adminstrator of each server. But each DNS server is run by either a government entity (or semi-government, such as a university or a government run corporation) or a private telecom company. And as such, each entity that runs a DNS server will have TOS (terms of service) that dictates the use of that server.
And each - I'd assume - forbids DNS access in for form of high-volume, repeated accesses such as would be used in a DDOS stress test.
You should find the administrative domain and website of the DNS server you want to use - i.e., you mentioned Indonesia, so
telkom.net.id, etc., from
public-dns.info - and read their TOS to determine their policies about access to their DNS services.
But there are many other factors involved in a DDOS stress test other than the one DNS service: you need to consider each ISP, each upstream provider, each network between the DNS service and the target server, in each country, as a DDOS attack - even a controlled stress test - creates huge amounts of traffic all across each network and as such costs time and money to each. And each services' TOS may very well forbid such use for a DDOS stress test.
So even though you have the approval of your boss for the test, I doubt very much he has approval from all of the necessary networks to use their services for a stress test.
In the US at least, DDOS attacks are illegal: https://www.law.cornell.edu/uscode/text/18/1030#a_5
AFAIK, there is no global law regarding DNS server abuse; governance of the DNS system falls under ICANN https://en.wikipedia.org/wiki/ICANN