IAAL but TINLA, but you should see an intellectual property lawyer and ask for their advice on the following matters. These things do vary by jurisdiction, although some of it is based on the TRIPS treaty (required for WTO membership), so it is getting to be less different between the jurisdictions.
Firstly, if you did this as a contractor, you quite likely still own the copyright, unless you signed an agreement saying you don't. In that case he client has a licence, the scope of which may vary, but not so far as to allow them to apply their own copyright claim to the exclusion of you. Secondly, what they have done is quite likely a breach of your moral right of attribution, especially if you were a contractor rather than an employee.
There may well be scope for a nice scary letter from a lawyer to get them to behave.