Five day old kitten losing weight!

As a few of you know my cat Izzy gave birth to five healthy kittens. I have been weighing them every day and the cats seemed to be doing well however one of the kittens have lost 3 grams over night instead of gaining, he is suckling and getting milk, I was wondering whether I should wait to see if he gains weight by tomorrow or start supplementing him now?

I am far from an expert about kittens.  But I'd start supplementing now; kittens can go downhill fast.

Was this kitten the lowest weight/'runt' of the litter?

I was thinking that however the vet is insisting to leave it a day minimum as she said shed only want me to supplement them if necessary. So im at a loss, the kitten does get milk and he is satisfied not mewinf all the time and he does look plump he just looks shorter. Xx

   I don't like to see weight loss in a baby kitten,  but   I would give it another day and try to make sure he is nursing every few hours.   Try putting him on one of the back nipples  (closest to her back legs)  since they usually are the "best" ones that have the most milk.   and watch to make sure that other stronger kittens are not pushing him off.    

Make sure he is not crawling off by himself and getting cold because then he can't digest .    (not to mention he will get lethargic and even hypothermia if he gets too cold. )

 So really make sure he is staying warm.   You could try heating a sock full of uncooked rice in the microwave  and put that by him if he is not snuggled up by mom    - or even if he is, put it on the other side.  ( Of course don't heat it so much that  it is HOT ) .  

Also check his mouth  make sure it looks normal , no problems like cleft palate.    ( this is unlikely  or I would think it would have been a problem  from the start.  but it can't hurt to check if you haven't already done so. ) 

Does his color look normal? 

Any chance he could be vomiting? 

 Is it possible there could have been an error in weighing him the previous day?    Either a problem with the scale,  or perhaps  2 kits who look very much alike and you weighed one twice?     I'm just trying to think whether there might be another explanation  so that it wasn't really a loss after all. 

Through out the night and day I check on them, make sure there all nursing especially this little kitten, he does seem to struggle to find a nipple but once hes on it he is nursing for a good while.
Last night izzy got out of her bed and slept in my bed without her kittens until I woke up and moved her about 2am this morning.
The kitten looks normal in himself. I do try and latch him on the bottom nipples but he prefers the top for some reason.

There was no mistake with the kittens hes the only red kitteb I have and the scale is brand new with no faults. I will just have to keep an eye on him x

 To tell the truth most consumer scales are not absolutely accurate to under 2 or 3 grams anyway.  so it might mean he didn't gain but didn't really lose either.    and i've had some who don't gain one day then the next day they've put on 20 grams.  

 so just keep an eye on him and we will think good thoughts  / keep him in prayers and hopefully the little red boy is  just fine .    

Just a little update my red kitty has gained 15 grams so hes back on track xx

 Yay Red Boy!    I am so glad to hear the good news.