Severe weight loss in rabbit - please help

I have a family of four rabbits all of who lost a lot of weight due to extreme decrease in weather over the last weeks but all of them have managed to put on all the weight except my 2 year old male. He still eats and drinks but isn't gaining weight, i've checked him for skin infections and visible parasites and can't find any. Now he can barely stand and just lies down, but does still make an effort to eat and drink. I can't find the reason for this online anywhere and all the vets in my area are useless when it comes to rabbits. please help

You say he is eating - a full meal or only picking at food? Are they fed separately?

today he's been eating carrots seperate from the rest because of how ill he is today (but they usually are fed ruffage) but he is maybe taking 2-5 or more bites out of it and then stopping to rest before repeating. this morning he couldn't lift up his head for more than a second but currently he's able to but he still isn't standing up.

Definitely not good! Get critical care and liquids into him quick. Any vet should be able to check for worms...you’ve checked his feces for worms? Are the rabbits fed pellets at all?

We've checked and there aren't any worms, but we don't think it is a parasite since the others would more than likely have it. Their occasionally fed pellets but the majority of their diet is ruffage, grass, fruits and vegetables. He is doing much better this morning and is now eating and drinking properly, able to sit up and has regained a lot of energy since the other day. however he still isn't standing up on his own yet.
Thanks for replying btw

What exactly do you mean by "roughage"?

Some rabbits are much more sensitive about what they eat. For example, too much sugar can cause some rabbits to get a serious imbalance of bacteria in their gut which can cause GI stress. The general rule of thumb is no more than 1-2 tablespoons of fruit per day max.

Hope she continues to improve.

roughage is finely chopped hay with some other things in it (im not entirely sure of this rest). they don;t usually get too much fruit but i had no idea they were only meant to get that little as even the vets told me their diet should consist of a quarter of fruit. thanks.

Rather than this "roughage" why not offer actual hay? The long strands of hay are an important part of a bunny's diet. It should not be chopped. I see you also mention grass. Hay is dried grass. A rabbit should eat his body size in grass/hay every day.

Greens are next in importance. Think herbs like basil, cilantro, mint, and other greens like dark lettuces, spring mix, green tops of carrots.

Fruit is just a treat. Pellets - plain, healthy ones - can provide minerals and nutrients to ensure a balanced diet but aren't absolutely necessary.

I'm not sure what you mean by " extreme decrease in weather " but you need to get this poor little bunny to a vet immediately...look for a vet that deals with " exotics "...they are trained to deal with rabbits.

He's been getting twice the amount of grass as usual as well as a pellets bar that we got him a couple of days ago. I've cut down on his fruit intake and he's now been eating more greens such as rocket and spinach and thankfully has almost fully recovered and finally started putting on more weight. we're not entirely sure why he lost so much weight so suddenly but thought that it could have been because of a major stomach ache. thanks for your help

by "extreme decrease in weather" over here it was around 30-40 degrees for a long while and then suddenly started dropping down to around 17-18 and then was all over the place. When he first got ill i looked online for any vets near us that specialise in rabbits but couldn't find any, and many of them only have mild experience. luckily he is feeling much better now and has put some weight back on

Glad to hear he's doing better. This may be a good time to see if there are any rabbit rescues around that can recommend a rabbit savvy vet. The best time to look for a good rabbit vet is when you don't need one. That way you already have one when an emergency does come up.

This forum also has a vet recommendation section. If you don't see your location, you could post and ask.