Which fits the best here? blue vs low vs down
“He’s been ____ for a few days. I intend to see him and take some food there.”

See a translation

@handsomeish

The simplest way to incorporate those words is to add one word – feeling.

He’s been feeling low for a few days
We’ve all been feeling down lately

It’s much easier to say – blue – without placing feeling in front of it, although you can do that as well –

Is she feeling a bit blue?
Is she blue? She seems like she is.

To say that someone is blue, and feeling blue, is basically equivalent – blue is 100% understood as a feeling. Obviously however, both low and down have myriad other uses – so you have to add feeling.