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NJP is non-judicial punishment. In terms of the civilian world, it's not a conviction but rather, an administrative process. So, he wouldn't have any conviction or points for it.
That being said, it is a career killer in many instances. It can result in a reduction in rank, forfeitures of pay, extra duty and restriction to a certain area (these punishment levels differ based on the rank of the person offering the NJP).
It can also be placed in permanent records, which can effect the ability to promote (meaning that he would reach his higher tenure date and be separated if he couldn't get promoted). An NJP can also form the basis for a separation itself, allowing for an administrative discharge though there would normally need to be other accompanying issues to allow for that.
I can't say how bad it WILL be, because commanders have so much discretion. They can go the worst or give no punishment. I can just say what can happen.