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.
It should accept "He has lost 5kg" and "he has lost five kilograms" but it doesn't.
I suggest using jin(斤) instead, which is used more often than kilogram in daily life. In case anyone was wondering, 五公斤=十斤.
Slightly different from the usual translation, but can substitute 瘦 well; here, 减肥 means "to lose weight". You can simply write 减 instead of fully writing out the compound word.
- 他 减肥 减 了 五公斤
- 他 瘦身 瘦 了 五公斤
and without the “do exercise” sense:
- 他 瘦/轻 了 五公斤
- 他 体重 减/减轻/减少/轻/少 了 五公斤