Можно вместо healthier сказать more healthy?

Легко.

Two-syllable words usually can take either form: more healthy, healthier. The choice is often one of habit or style more than grammatical correctness.
BUT ))) "Healthier" is by far more common. If you want to prove it. Just google "more healthy" within quotes "" and you will get only 6,180,000 matches, while if you try "healthier" you will get nearly 80,000,000 matches. :)