Can our son enroll in an international elementary school in Thailand with a DTV visa without switching to an ED visa?

Depends on the school.

I am on retirement visa and my daughter not have ed visa. Dont need visa under 15 yrs of age

Going to school does not require any specific visa. So yes, your son can legally attend school while on a DTV.

The problem comes from the schools. They want your money. They want you to continue paying tuition every year. So many of them don't want students unless the ability for that student to remain in Thailand is dependant on you paying their tuition. So many will require you to get the ED visa through them or they will not enroll the student. It's basically blackmail.

Same thing happens to many people that are married to a Thai and have a marriage visa. Someone on a marriage visa can get a work permit, but many employers will not allow them to work on a marriage visa and instead requires them to switch to work visa, that way they are dependant on the job to be able to stay in Thailand. It gives the company leverage over the employee.

It depends on each school. Both my kids are on dtv and the school doesn't even know. Other schools may require the student to be on an ED

You need to ask the school