IP rights contract

Hi everyone,
I've worked with a client for quite a long time, on and off, they develop a facebook game so they needed content pretty much constantly, so we've been doing this for probably more than 4-5 years already. I was invoicing him and he paid me via bank transfer, we put all the details of the work in the invoices so we kept track of the work being done. We also have emails in regards to what was requested, feedbacks, iterations, etc.
However, he never set up a proper contract for this work and now he wants to do it, so that he can own the rights on the graphics (apparently due to someone trying to steal it and use it for a different game - which I've seen happen in the past). He was very good with payments and we kept a great relationship all through the time we worked, so I'd like to help him there, but I will not sign a contract that is back dated. He thinks it's the only way, but I am reluctant to do so. Is there any other way for us to sort it out for him? I certainly don't mind giving him the rights to help him be protected from theft (and own the rights) but at the same time I don't know what's the best way to do this. we're talking about graphics done over a very long period of time...

If anyone has any thoughts or advice, that would be very much appreciated.

Many thanks!
Virgil