I also think that newer porta cots and mattresses are made with SIDS guidelines in mind. That news article is 6 yrs old, and I've seen the difference in portacots from then and now. The newer ones have those breathable net sides that go almost to the bottom..
Anyway. Point is. I'm sure u'll have ur nephew's safety in mind when making ur purchase.. if he is used to a mattress, then so be it. Maybe his parents have a preference to the type of mattress you'd need?