Sealed does not mean it did not happen. The government may well have access to it and discharge him for lying about it. He should disclose it to the recruiter and ask for advice on how to deal with it. But the military wants to know and does not want to be deceived and the chances are they will find out at some point.
This response is for informational purposes only and is not offered as legal advice.
Having a sealed juvenile record is suppose to be deemed as though it never happened. However, when dealing with gov't agencies, it gets a little trickier.
I had a client who had a juvenile record that was sealed but did not qualify for the marines, army, or air force but did qualify for the navy.
I think the best thing to do is to go talk to the recruiting officer as mentioned and see what they can do for him.