Do you have a kitchen or postal scale? If so, weigh him and let them know how much he weighs. If he's losing weight they might want you to tube feed him. It's really very easy and safe if you can go slow and follow directions.
-Kathy
That dose is well within the dose ranges that I have read about, so that's fine, but it will *not* treat any secondary infections like e. Coli, which I suspect he has. Please, please, please call them and talk to them about treating for a probable secondary e. Coli infection. A few of us here...
Ask the vet for:
metronidazole for the histomoniasis (blackhead) - dose is no less than 30mg/kg once a day for five days.
Baytril, Clavamox, or something to treat the probable secondary e. Coli infection
Also have them sell you supplies for tube feeding and have them show you how to do it...
Drugs are almost impossible to get in the UK, but a pigeon breeder, if you know of any, might have what you need, so start looking for a pigeon breeder in your area. Canker in pigeons is treated with the same medicine, that's why I mention it.
-Kathy
If it's blackhead he will need metronidazole, a wormer that kills cecal worms and probably an antibiotic like Baytril, cipro, Clavamox, augmentin, or SMZ/TMP. He will also need to be brought indoors and tube fed. I have had to treat more than my fair share of peafowl with blackhead, it must be...