Can a 9 year old boy wear no shirts in the swimming pool?

It depends on the customs of the country. In the US, it is very common from boys and men to swim bare-chested, wearing just swim trunks. Girls and women, however, are expected to cover their chests and wear a bathing suit. Of course, in some cultures, even in the pool, people of both genders are expected to keep their body covered; so find out the local customs by asking. But if you are going to a typical US swimming pool, a boy would bring a shirt for when he comes out of the pool, but he usually does not wear one in the pool.