Reckless endangerment is an extremely serious offence especially when he not only endangered fellow Starfleet officers but civilians as well, if anyone had died, he could be looking at murder or manslaughter charges as well.
Personally, I'd recomend six months to one year brig time (maybe more), demotion by one grade or perhaps even a dishonourable discharge. From what you have described there was no call for him to do what he did, there were no mitigating circumstances. When called for you have to throw the book at them otherwise they think they can get off lightly everytime they screw up badly. This time nobody was killed, next time many may die if their cavalier way of dealing with problems continue. Nip it in the bud before they really do something that may end up with thousands of casualties and a lost starship.
Arise, arise, Riders of Theoden!
Fell deed awake: fire and slaughter!
Spear shall be shaken, shields be splintered,
a sword-day, a red-day, ere the sun rises!
Ride now, ride now! Ride to Gondor!
Theoden King: The Return of the King