The modal verb "will", and its past-tense (that's "past tense" in the purely technical sense) form "would" can sometimes indicate habit, especially when there is stubbornness involved. In the present:
He will always take the bus, even though the train is much more convenient.
And in the past:
She would always turn the TV off at the most exciting moment -- that's why I left her.
We can actually omit the word "always", but we have to be careful with the intonation. Usually we stress the modal verb:
He will take the train...
She would take the train...
(Excuse me if this post seems a little disjointed; my wife is deliberately trying to distract me because, as she has just informed me, she likes to annoy me. In fact, I could say: She will insist on asking me questions about the first time I kissed a girl...)
In these sentences, the speaker clearly disapproves (now my wife is telling me how her comfortable shoes are falling apart).
In Tdol's sentence, we have a similar feeling of disapproval, except that the speaker disapproves of the fact that he doesn't eat better. The modal verb "would" expresses the idea of a habit, while "if only" indicates a desire that is unfulfilled.
Compare:
"If I had a hammer" (but I don't); and
"If only I had a hammer" (I really, really need a hammer, but I don't have one).
We can move the word "only" to a different position:
"If I only had a hammer."
Finally, the past tense form "would" is used not to indicate that the action should have happened in the past, but to indicate that the action is hypothetical -- that is, that the action never took place and most probably never will:
"If he would only eat better..."
That means the same as:
"I really wish he had a habit of eating better, because then he wouldn't have so many health problems."