Symptoms with strange speech and defiance?

I have a family member who is 3 and will be 4 in July. His speech is strange. Some of it is understandable, but most of it is like a made-up language he's developed. When asked what he said, he repeats the same syllables. He's very defiant and when asked to do something says, "I can't. It's too happy." He's very obsessed with Hot Wheels, but I don't know if that has anything to do with his other behavior. He has hundreds of them and asks for more every time he is at a store. When he plays with them, he gets ALL of them out and fills tables, chairs, and the floor with all of them. When socialized with kids his age, he follows them around reenacting scenes from movies with his version of English/made-up language. He still is not potty-trained, either. He figured out, though, that he receives a treat for going in the potty. So, he went in his pants and put the "waste" in the potty, thinking he'd get a reward. He seems affectionate, but he doesn't seem concerned when he accidentally hurts someone. I've urged the parents to have his speech checked out, as his mother is due again in just a few weeks and will have her hands full then. Thoughts?