1. If I would eat better food, I would be healthier.
2. If he would eat better food, he would be healthier.
#1 sounds odd to me not for grammatical reasons, but because to speculate upon your own willingness to do something as if you had no say in the matter seems odd.
#2 sounds fine to me; I would paraphrase it as "if he were willing to eat better food, he would be healthier".
What about this one:
3. If I would eat better food, I would be healthier? What on earth are you talking about! I eat perfectly healthy food!