We like to sleep in a cold room with warm blankets. We sleep with fleece sheets, a thermal blanket (lightweight cotton with a loose weave with air holes that capture air for insulation when there's another blanket on top) and then a down comforter.

But I'd encourage him to also use a space heater. They are very safe these days.

These are the sheets we use now: https://www.amazon.com/gp/product/B01MYWIHWZ/ They are very cozy though I think they help the most when you first get into bed (no icy cold sheets!)

For a thermal blanket, something like this: https://smile.amazon.com/Glamburg-Blanket-Breathable-Thermal-Blankets/dp/B07MB9KKSB