The key to finding the answer is knowing his name at birth as that is the name that will appear in the birth index. You may well have seen it already but not recognised it as the name will be unfamiliar. As others have said there are organisations that can help relatives of adoptees as they can access records (such as the Adopted Children's Register), that are not open to the public.
The last column on the birth certificate should note an adoption took place but nothing more than that. There will be nothing indicating adoption within the birth index itself and any name change that takes place as a result of adoption does not appear in the birth index.