Translation:He lost five kilograms.
Please be more consistent in accepting (or rejecting) present perfect. This can mean "he has lost 5kg" as in 他一直在减肥，瘦了五公斤 (he has been on a diet this entire time, and has lost five pounds) or it can indicate that "he lost 5kg" as in 去年他瘦了五公斤。今年他吃的更饱，又胖了点儿 (he lost 5kg last year; he ate better this year, and gained a bit of weight). In my opinion, either translation is perfectly acceptable, and is heavily influenced by context.
可是如果你把“他胖了五公斤”给翻译成“he has gained 5kg"你起码得接受“he has lost 5kg”作为“他瘦了五公斤”的英文翻译啊。太不一致了，Duo。我后悔给你那么多的Lingot.