My 2.5 year old has lost weight and I'm really worried.

I feel like the worlds worst mother.
DS was really sick yesterday, with temps of 39.5, refusing to drink and eat for over 6 hours, and the occasional shakes, so last night on advice from nurse on call we took him to the hospital.
He finally had a hydrolite (?) icypole and his temp crept back down, so he was allowed to come home, but there was talk of admission for fluids.

Anyway, when we got there, they weighed him, and to my horror, he was 12.45kgs. :o
At 23 months, he was 13.2. Hes lost around 800 grams in 6-7 months and with that, from nearly 75th percentile to nearly the 10th percentile.
Um.... should I be as worried as I am?
He is super active, a super fussy eater but his habits haven't changed for quite some time, and I haven't noticed him eating or drinking less.
Do I take him to see someone? It just seems so small for a 2&/2 year old when his parents aren't small people.

If he was a bit dehydrated that would account for the weight loss. You would be surprised as how quickly they can loose weight through fluid loss, especially if they have a fever and aren't taking in fluids through the day. I think you will find that once he is well and eating and drinking normally again, that the weight he lost will come back on. As for his overall weight I really wouldn't worry. My DS is 2.5yrs in 2weeks and he is only just 13.5 kg, maybe 14kg now. But like your little boy he is super active and isn't the best eater in the world, he's just a toddler kwim? But if he doesn't gain that weight back he lost from being ill, then maybe just keep an eye on his food intake for a while just to make sure that he is eating enough and maybe slip in a few high calorie things like butter, cream and stuff like that to boost him a little.

I agree with Trillian
I would not be overly concerned. That weight loss is equivalent to about 3 glasses of water. Even if he is down to the 10th percentile it is still within acceptable limits. I would just keep an eye on his weight over the next few months and see how things go.

When my DS had swine flu last year he lost about 2 kgs in 2 weeks. Once he got better he gained it back (plus more) in no time. Perhaps just check in his weight in a few weeks.

HTH

The others are right. Single, spot weights are pretty useless in terms of assessing nutrition, especially when one of the weights was when he was sick. To get an accurate idea of his weight, you need to weigh him naked, once a week, for a few months and see what his weight is doing.

I would be largely unconcerned about a 2.5 year old that was down 800g from a previous weight admitted with dehydration, and assume it was simply fluid loss. Last time I had gastro, I lost over 2kg in 24 hours.