Has he ever had his hearing assessed? This can explain developmental issues which involve speech and behavior, of which he sounds as if he is at the developmental level of half his age. This doesn't sound like ADD, -but it does need immediate attention. Even children close to age 5 who are isolated from others with only adults around should be speaking short sentences and comprehend more. For example, my daughter (an average child) is a single child, but was able to communicate with simple sign language at 9 months of age. Even though she couldn't speak yet, she could still comprehend words.
I fear for this child as he will have a difficult time once he enters school if his issues aren't addressed. You didn't say if his parents are involved in his life, or if they are "quality contributors" (no offense, but some are caught up in drugs or careers and ignore their children). You are right to be concerned...~MM
Well, clearly it is time to seek help with a speech pathologist. He seems to have difficulty with receptive and expressive speech elements. Why he is having difficulty would be the question. Is it a speech delay or secondary to something like sensory integration disorder in which his motor planning is not functioning properly? But you need an expert to evaluate him to discern which.
Since you are his aunt, it is a good question as to what his mother says about it. Is she interested in pursuing an evaluation? Once he is in elementary school (kindergarten) then the school will make screen him and pick up on the speech issues as well. At that point they will get involved and make recommendations as well.
Kids with great difficulty in speech also have social issues. Try to foster an atmosphere of patience with your boys when they are together with their cousin. I'd say that you must supervise quite a bit and help out. But if you aren't understanding what someone is saying and you can't convey your own message, that is probably very frustrating and you'll switch to another tactic.
Hopefully he can get a speech evaluation relatively quickly. Good luck. Oh, by the way------- how are his gross and fine motor skills?
Yes, how does he do in school or preschool? You might ask his mom about his speech. See how he talks when he is with his mom.
Is he of normal intelligence? How does he fare at nursery school? What does his mother have to say about his speech? You didn't mention any speech impediments, like stuttering or lisping, only that his speech was limited to one word responses, and that those responses often made no sense. Is it possible that the boy is uncomfortable with you, and behaves oddly because of it?