As your kiddo is growing, his ability to understand sounds better is improving. Now he knows what sound a word produces and can tell you a word rhyming to the one you have just said.
What you need to know:
As your child knowledge of phonetic sounds are improving, it is your responsibility to help him learn the concept of rhyming words. Keep asking him for words with similar sounds, you can ask him what words sound like cat, he may say rat then continue saying, cat and rat sitting on a mat. This fun way of learning with you will make things easy for him.