Is it a red flag if my partner never expresses any boundaries?
Possibly. People have peanut allergies. People have bad circulation. People have migraines. People have bad days. Whether your partner likes it or not, human bodies and human circumstances prescribe boundaries of their own.
For what it’s worth, it’s important to distinguish between boundaries of thought and of action. Some people have nasty fucking imaginations that encompass all the world and every hell below, but would never dream of actually, say, gutting three of your stuffies and turning them into a turducken of sadness.
If this is currently a long distance thing, he may be living in his head, where he may indeed have no significant boundaries. So ask him specifically about later, when you’re physically together… what are his priorities, what are his concerns, and so on. If you still get nothing, I’d say we take those flags straight from red to black.
In that case, I’m afraid he just doesn’t care enough about himself to care about you at all.