How can I reassert dominance with my dog?

Easy answer is to put a collar on him that he cannot slip out of. Dogs will learn quickly that slipping collars means freedom. The only way to fix this is to use a collar that he cannot slip out of. Your more attention needs to include retraining the recall. I don't see this as a dominance issue as much as a lack of control issue. He's probably acting up because he now has a buddy.