You're right, your baby isn't doing well right now and showing symptoms that don't point to any one specific thing, but could be quite a few illnesses. This is why you can't waste any time trying to fix this at home - he, needs you more than ever right now
Let's get him supported until you can get out the door with him. He's going to need supportive care while you arrange seeing his doctor - today. Today. Yes?
For a makeshift brooder, use a small box lined with soft clothes like tee shirts.
Use a thick, clean sock and fill it ¾ with plain, raw white rice. Knot the end and microwave it for about 1 ½ minutes. Shake it afterwards to distribute the heat and be sure it's not too hot. Tuck this in just under the cloths.
A heating pad under the box is also helpful, set on low. This is one of the few times I’d ever use both heat sources if necessary to maintain incubation temp (90-105 degrees).
If ever using an electric source for heating anything in anyway, please be vigilant and constantly double checking carefully.
Gently drape a light cover over this box to further help hold heat in and keep light low.
You may offer a few drops of sugar water to the side of their beak with your finger or by eye dropper; or a dab of corn syrup, maple syrup or bit of natural jam/jelly to help with blood sugar levels.
I wouldn’t force fluids or food though until a professional has agreed it’s required and shown you how. The last thing we need is aspiration pneumonia.
A bird may show their weakness, illness and lack of energy one moment, even for a few hours or days - and then “suddenly” seem to be fine. This is their getting a second wind. Finding the strength to ‘mask’ the illness or problem. And since this masking can continue for a while, the underlying issue is only getting worse. The next time you see the bird acting ‘off’, it might be really, really bad.
If your bird is acting ‘off’, no matter what – no matter when, they need to be seen by their vet. Infections and disease are far more successfully and inexpensively treated when tackled early. Unlike mammals, avians don’t fare well with a “watch and wait” protocol.
Though specialized avian vets are ideal, any vet who sees a majority of birds or at least 1/3 of their practice consisting of birds is a good choice.
I'm not sure if these options are available in the UK, but if you have a Pet Smart, you have Banfield Clinic inside, open 7 days a week and they see birds.
If you have a Pet Co, they have a list of vet names that they use for themselves and are happy to give you, ask for 'bird vets'.
Ask any vet in town who they'd recommend for bird care. Ask any good breeder in town who they use (if they don't use anyone, they are not a good breeder, stay away from them).
Please, please don't delay, ok?