Though linguistically speaking, "mefus" is "collective" and "mefusen" is "singulative" (rather than "singular").
It's one of those Welsh nouns (usually for things that often occur in groups such as some plants... or children) where the basic form is collective, and you add an ending to get the singulative form for one of them.
Sort of like "rice" versus "grain of rice", where you add something to turn the collective into "just one".
No, "he" is just e.
Word for word, you could translate this as "Is he (in) want (of) strawberries", with the words in parentheses not present in the Welsh.
But perhaps best just to learn that Mae e eisiau... is "He wants ...".
Welsh usually forms the present tense with a form of bod "to be"; thus "I like coffee" will start with words that, by themselves, mean "I am".
(Consider the English equivalent "I am liking coffee" which also starts with "I am". We don't usually say "He is wanting", but perhaps you get the idea.)