If he has completed all requirements to get a license and the DMV would issue him one if he went down to the DMV and showed proof of completion of all the required classes then his suspension starts then. If he doesn't have any of that complete or he hasn't informed the DMV of this then the suspension will not start till he is 18.
Legally eligible, in short, means that there are no reason why he couldn't go down to the DMV and get a license immediately. If there is something legally stopping this, then he wouldn't be legally eligible. If you or your son has questions of when exactly he can get his license, and what he may need to do in order to get it, I would recommend contacting the DMV directly. They should be able to pull his record and advise accordingly.
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