Even now has two meanings
with a verb in the 'simple' construction it means ‘still, in spite of [some circumstance]’
Even now, after all he had learned, he believed in her innocence.
with a verb in the progressive construction it may have that meaning, or it may mean ‘at this very moment’
He is pleading even now, even as we speak, with the Prime Minister to pardon her.
But even now is not used with a perfect construction in either of these senses.
I suspect what you mean is one of these:
Even now the boy's body is being devoured by disease.
Right up until now the boy's body has been devoured by disease.
In either case, the adverbial phrase is a sentential modifier and may be moved after the first auxiliary (is/has), after devoured or to the end of the sentence. It's permissible to move it after the second auxiliary (being/been), but I find that pretty clunky.