How about the Norfolk Broads ?? Plenty of places to see wildlife au natural, or in zoos, like Banham Zoo which we thought was excellent. Also seals at Blakeney etc
Discovered this site which apart from having some excellent bird photos your son may enjoy, also offers customised bird watching trips which may be of interest.......(I understand that they have a relative with Aspergers Syndome which may be helpful)
http://www.robwilsonphotos.co.uk
Otherwise what about somewhere in the vicintity of Slimbridge?
http://www.wwt.org.uk/centre/122/visit/slimbridge.html. I remember many years ago staying at Slimbridge YHA to visit the wetlands centre - the hostel had a largish pond in it's grounds which was full of ducks etc