Mel
DD is 2.5yo and 13.8kg. She lost about 1kg in 6 weeks before her 2nd birthday but that was after being sick for most of that time. If your worried take him to your gp/paed for a check. I did this with dd1 and she checked her fortnightly for a while and she regained the lost weight.

As others have said, different scales will give diferent readings, if the same scales are on different surfaces or uneven surfaces you can get diferent readings. If he is done a poo or jsut eaten a big meal (although if he is anything like my dd a large meal would be a miracle lol) and the time of day can alter the readings on a scale. Try and weigh him same time of day, same scales and on the same even surface ( we dont put the scales on the tiles anymore as they can vary by 1-2kg). Good luck, life with a fussy eater is not easy