"Jednego" is the Accusative version for the masculine animate (see here) - the thing is, that some nouns tend to be grammatically 'animate' for no reason at all.

The people in the comments above yours were arguing whether it should be "Mam jeden cukierek" (treating it as non animate) or "Mam jednego cukierka". One of the links says that Polish language has more than a thousand "exceptions" - that is, words that can be grammatically animate for no reason. Seems that many of them can be treated as both depending on your preference, although it's probably more colloquial to treat them as animate despite what they really denote. For example, the Polish wiktionary only puts the version "cukierek" as Accusative, while the English has both of them.

In this particular example, the animate version "Mam jednego cukierka" sounds a lot more natural to me, even though there's not much logic behind grammatically treating it as animate. This is a difficult issue and unless you find somewhere a list of such 'illogical' masculine animate nouns, I guess the only way is just to observe the language in use.