The CBSE board has a best of five rule in which your main percentage is decided by one language subject I.e. English and other 4 subjects in which you get high scores. And the remaining subjects becomes additional whose marks won't be added to your main percentage.

The subject you would score lowest will automatically excluded . So, if you score higher marks in additional subject than any of your subject other than English then it would automatically replace the lowest scoring subject.

If we talk about your son's overall percentage then we will exclude the marks of Bengali subject because he scored lowest of all subjects in Bengali.

Percentage = 484 / 500 * 100

That is 96.8

Hope this will help you !