If you want your son to get admission into sainik school and he is in 6th class .then admission will not be done because sainik school admission test is conducted only for 6th and 9th class.

However if your son is willing to get admission then he has 2 options either he Get admission and repeat his 6th class in sainik school or or he can give admission test in 8th class  and get admit into 9th class in sainikSchool.

Sainik School Admission 2022 - The National Testing Agency (NTA) will release the AISSEE 2022 application form in October 2021 (tentative). Students can complete the Sainik school admission process online from aissee.nta.nic.in. To submit the Sainik school admission online application 2022, students need to upload all required documents.

Eligibility for Sainik School admission 2022 for Class 6

Boys whose age is between 10 and 12 years as on March 31, 2022

Students should be studying in Class V in a recognized school

Eligibility for Sainik School admission 2022 for Class 9

Boys who have an age between 13-14 years as on March 31, 2022

Students should be studying in Class VIII in a recognized school..

https://school.careers360.com/articles/sainik-school-admission

Good luck