"A fish" would be "un poisson." Du is like "some" or an unknown quantity. Here is a helpful link: http://www.languageguide.org/french/grammar/articles/
They sound actually different. "IL" sounds like ill in english, and "ELLE" sounds like the first part of elephant in english (until the first 'L'). What is actually hard to differentiate is the singular and plural forms of each, e.g. IL ans ILS sounds the same, in the same way ELLE and ELLES sounds the same. The only way to tell a difference is by hearing the full sentence where is used. Greetings.
"il" and "ils" sound exactly the same, but the verb "avoir / to have" conjugates differently with each subject; and that tells you whether the sentence is in singular or plural form.
he has fish -> il a du poisson.
they have fish -> ils ont du poisson. (in this sentence, I believe, you have to pronounce the 's' in "ils" as the next word starts with a vowel)
To be honest, it's very hard to translate this French sentence; as it is in singular form, whereas, the English translation you provided would be understood in plural.
The latter is a better translation, although I will still understand it in plural; as in, he has more than one fish, but I don't know how many, so I'll say some.
The French sentence means, "he has some part of a single fish".
Hope that makes sense.
First of all, the verb is "avoir", not "manger", so "eat" is completely incorrect. Plus, "du" (meaning "de" + "le") used in this context means "some" or "an unknown quantity of", not "the fish". "He eats the fish" would be "Il mange le poisson", or assuming you mistyped and meant "He has the fish", that would be "Il a le poisson."