That has happened with both my sons but the second one has had lots of medical problems that seemed to exasperate (sp?) the issue. He went from the 50% to off the charts (not even 1% - like way off) by his first birthday. We did testing for him because he had all these other issues, but he is fine overall. He just has bad acid reflux and breathing problems and seems to have a difficult time retaining weight no matter how many calories he was consuming because he needed more in general.
Most of the time it seems to be nothing to worry about. I did realize the doctors concerns with him though because he dropped so much off his weight curve. They actually are more concerned with how they grow in relation to their own growth curve. When there is a drastic drop, it can signal a lot of issues, but oftentimes the kids seem to be fine. My almost 4 year old is just hitting 32 pounds and my now 2 1/2 year old just hit 25 pounds and the doctors have calmed down about the issues.
(BTW both my kids had wonderful appetites). You can always give him things like Greek Yogurt (not the 0% fat kind but the full fat kind - it's 15 - 18 g fat per cup) or Pediasure if you are really concerned, but otherwise a lot of times if you wait it out, they will be fine:).
Good luck!