He was just 4 years old when he was told that he was incurably ill and had only a few months left to live.

Is “left” redundant?
Could someone please help?

Hi. I, not a native speaker of the language, mind you; do not think it's entirely redundant, as to my ear it kind of helps assert said few months are all that can be gotten - it's the estimated prognosis for his health condition, and it's an end that can't be escaped.

Without left, and with some stretching of the mind, one could argue, for example, that the doctors where not eyeing the real demise of the patient, but were rather referring to some point in the expected outlook for someone of his health condition, where he will, say, become incapable or disabled beyond the point of being able to live on normally - think of Alzheimer's. In that sense, those few months are not really all that's left, but is what makes sense or matters according to some criteria or point of view .. .

Anyway, and egardless of how much sense the above reasoning makes, note here that that statement is reporting what's been said by others, so the decision of omitting left from there is not readily authentically valid, since those were not really the speaker/writer's words, and they're merely relaying them here the way they were originally said. The statement is fine.