He is the registered owner of the domain name, can he re-launch the site without me?

Yes, he can act as if he was sole owner of the website, and no, your rights to the website contents wouldn't affect use of the domain name itself. Your contract rights for the website development (and its contents, of that's what's in that agreement) are separate from your rights vis-vis your domain name's registrar, and your regsitrar may have no idea you exist if your co-party to the contract registered the domain name under his own name as registrant. The registered owner, or registrant, is the legal owner, as far as the registrar (GoDaddy or Register.com or whichever registrar you used) is concerned. Please see my Legal Guide on ownership of domain names or see the referenced link.

Without seeing your contract with the website developer, I can't tell whether or not you and your co-party signed as individuals or as partners or for an entity --it sounds like you might have a verbal partnership agreement, and you might have been able to register your domain name under that partnership name to avoid this happening, but usually without any restrictions in writing, basic partnership law says that either of you can act individually to bind the partnership. The fact that you paid for the website developer doesn't necessarily mean you both intended to share ownership in the domain name. A written partnership agreement could cover who owns what, and assets like domain names and website contents should be assigned to the partnership with restrictions on a partner's right to do anything to those assets without the other's consent.