Extension of stay based upon studies or dependency

My son 15 year's old are studying full time here in Thailand, in an international school and we are now about to do an extension of stay for the 3rd time within a couple of weeks, the first two extensions based upon initially a one year ED visa was granted a permission to stay for a year,  He changed school 2017 and now I'have heard that the school he attends only are able to provide the students with 90 day's extension at a time  witch seem to be odd due to the fact he attending school 8:30-15:30 every day with exception of Sonkran holiday and summer holiday July and August. Or should I put him as a dependent child based upon my extension if so, what documents are required?

Any advice are greatly appreciated