The trick is to keep everything in past tense except the word now. This is how it's done in published books.
Ancestor by Scott Sigler:
On the screen, a man crawled across the floor, leaving behind him a glistening trail of puss, blood and other fluids Chapman didn't want to think about. The man's once-white lab coat was now wet-pink, clinging to his body like a thin straightjacket.
Warbreaker by Brandon Sanderson:
The straw figure returned to his window. It carried a large ring of keys. The figure’s straw feet were stained red. The crimson blood seemed so dull to Vasher now.
(Examples chosen for no reason other than I happened to have a PDF on my hard drive for an easy Ctrl+F!)
Seeing how it works?
If you say...
Jeff was six when he first discovered he could play trombone with his feet. He had come a long way since then — he played four nights a week at MacLennan's, now — but life had only gotten harder.
...as your readers, we immediately get that then refers to the past of your story, and now refers to the present of your story. See how everything except the now stays in past tense?
It works because of what's known as free indirect style. Modern readers are completely used to this. We intuitively understand that we're reading the character's inner monologue, so we understand that the now means the 'now' of the story, as the character is thinking it.