I don't think it is a Asil.
Could be some type of Thai, Shamo or Brazilian.
How big is he? Weight and Height.
There is a lot of Oriental Game Fowl and the Weight and Height would help...
Edited to add
I just noticed that you were across the pond in England. It might be a Kulang Asil there a Asil over 6 lbs and I don't thing we have too many here in the States...
Ill ask my brother to either measure and weigh him or take a better pic next to something to get a size comparison ... Dont think the cock will let him get a tape measure next to him ..though i could be wrong ..
He does keep a few asil and this does look similar ...but then it also looks like the shamo's he has too ... Hence the confusion of what it actually is
Looks to me like a Thai/Shamo or Thai/Aseel cross. The tail is quite large, maaybe Brazilian in there too, but the body is so upright like a Shamo. The color also says Brazilian cross.
Gorgeous bird though! I would LOVE to have something like that dropped off at my place.