Hi my girl had a litter on Tuesday. Sadly 2 were dead but we have 2 alive ☺️. The girl is growing well and putting on weight but the boy is tiny. He's just not putting weight on at all. In fact he's lost a couple grams. All google brings up is FPS

Have you checked for a cleft palate? If there is no cleft a vet check would be a good idea. Stay with him 24/7 and ensure that you keep plugging him on to a teat especially if he has trouble latching on. Others will have tips too, I hope it goes well.

I would look into tube feeding him.  There are many great videos on YouTube showing you how.  It is not as scary or hard as it may at first sound and it gets the nutrients right where they are needed.

Thanks for the reply. There's no sign of cleft palate. He stays on mam and suckles well but I've bought royal canin formula to up his feeds

From being 131g at day 2, he went down to 124g. Ive been using puppy stim and ensuring he latches on to mam constantly he's gone up to 128g. His last feed was 3 hours ago (royal canin) and he wont take anything from mam or bottle since. I want to use the vets as a very last resort as i dont have much faith in them.

It's normal for pups to lose a bit of weight in the first 2 days after birth - it is a well-known 'thing', I can't recall why exactly - but many pups do.  But, after the first couple of days, you should then see a good increase.... So I would be watching carefully what happens from now on, and supplement if you feel you need to.  (But do stop supplementing when you feel he's doing ok too - mum's milk is best.)

So I've started puppy on Betty Bloomfield Rehydration at 5:40pm tonight and sticking to the drip every 20 mins. I read a fair bit on it before making the recipe and everyone had great comments on it. I'm just wondering how long its taken anyone to see a change in pup (he just wont latch on to mam now)?

I really think you are doing too much, too soon.  He may not latch on because he's full with all your supplementing, now... or because he's used to getting it the easy way in the bottle.

I would weigh him twice a day and if he loses weight 2+ days in a row, then is the time to supplement.  IMO you are rushing into supplementing too soon, when there may not be a problem - and running the risk of creating one...

^