Hello TakuLassa 

Whatever be the percentile of JEE Maisn there are certain eligibility criteria of getting into NITs. One of those criteras are:

Getting 75% marks in boards or be in top 20 percentile of your boards.

If you havent scored 75% marks then see whether your son is in top 20 percentile in his boards. 

If he fails to qualify this criteria then he cant get into any NITs.

But for ST its 65%. In that case too he hasnt qualified so ask your son's board whather he is in top 20 percentile in his board.

As far as marks are concerned then under ST category he can get into NIT Arunachal .

Hope that helps!