I don't think it is set in concrete  the age a child should begin to talk , every child is different. You say your nephew dances to the Wiggles on TV, I am wondering if he can actually hear the songs and music or is he just reacting to what he is seeing. Maybe a hearing test could be  the way to go.

Does he respond to what his Mum & Dad ask of  him, eg. pick up a toy, would you like a drink. If  he does, your nephew just might not be ready to talk.

It is nice that your so concerned for your nephew.

Well it is fairly common....  & so not really something to worry about.

But it can be a bit of a nuisance when they still can't clearly express what they want. Mostly it means they just haven't yet felt the need to give it a go. Some kids get by quite happily with grunting & pointing for years. One lad I knew put it off until nearly school-age;  he had an older brother who used to 'translate' for him. He's got no learning disabilities etc; he's just naturally lazy... Now he's a young man & has a (fairly sedentary) don't-need-to-think-hard job.

Around that age-group tho, I do know a good way to kick-start verbal communication:- Childcare. One with lots of kids & activities & little toilets, little play equipment, little tables & chairs, little beds etc. Then they quickly learn that those who can express exactly what they want/need, get it. Also the fully-qualified early-childhood workers will quickly spot anything unusual.... & its always nice for Mum to have a day off once a week!

I would start by suggesting that he gets his hearing checked.

Pay attention to how he acts too. If he can just point at the kitchen and his parents or grandparents will just get up and get him a snack and drink, well of course he's not going to bother to learn to talk and he's going to take the "lazy" way and stick to pointing and grunting.

My daughter didn't talk until she was 2 1/2 due to a hearing problem, but now a year later she speaks clearly.

 It can be normal, or it can be a sign that he needs extra assistance.  It shouldn't hurt to get him checked out by a doctor, though.

I think it depends on a few other things.  My now 4 1/2 year old son didn't really talk until he was about 2. And even then, he didn't speak clear words. He spoke a lot more and a lot clearer at close to 3.  But with my son, he understood a lot, even if he didn't talk a lot. I could give him 2-3 strings of commands and he could do it. For example, I can say, "please pick up that trash and throw it in the bin and then you can continue to play"

On the other hand, if there are other signs along with the not talking - for example, not making eye contact, not responding to names, etc... and things like that, then I think it's a good idea to get professional assistance.