The biggest heater in the store is not the one that will keep him warm. The one that works is the one that reaches the chair he sits in, the spot by the table where he reads, and plugs into an outlet that can handle it. Before you buy anything, measure the room he actually uses and the place he sits in it.
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Measure the Room He Actually Uses
Use the den or the kitchen corner where he spends the evening, not the guest room that gets used twice a year. Measure the length and width of the floor and multiply to get square feet. Note any unusual ceiling height, any window that leaks cold air, and whether the door to the hall stays open. Heaters are rated for a room size, and that rating assumes a closed room of ordinary height. A drafty room needs the heater to work harder, so either choose one rated a little larger or fix the draft first.
Read the Label, Not the Box Front
The box says warm, quiet, or fast. The label gives the heat output, the rated room size, the cord length, and the safety features. Look for a tip-over shutoff that cuts power if the unit falls, an overheat shutoff, a thermostat that holds a set temperature, and an auto-off timer in case he dozes. Check that the base is stable and that the controls are easy to reach without bending far. A short cord tempts an extension cord, which is the wrong fix, so cord length matters.
Match the Heater to His Chair
Stand where his chair is and picture him in it. Heat aimed at the wrong spot warms the air, not him. A convection or oil-filled heater spreads warmth slowly and suits a room he moves through. A unit with a fan moves heat faster and suits a chair he sits in for hours. Either way, keep it clear of curtains, bedding, and throws by the distance the label gives. Think about how he will move it, too. If lifting is hard for him, choose a lighter unit, or one that stays put on the floor.
Plug It Into the Wall, Not the Power Strip
Plug the heater straight into a wall outlet that can carry it. Do not run it through an extension cord or a power strip, and do not let the cord run under a rug. Check that the outlet is not warm or loose, and look over the cord for cracks. If the outlet is old, or the room has only one outlet for everything, ask an electrician to look before the first cold stretch. This is a job to ask about, not to guess at.
Check It Before the Cold Comes
Run the heater once in a mild stretch before you need it. Listen for buzzing, make sure the cord stays cool, and watch that the thermostat clicks on and off. Then mark its spot on the floor with a strip of tape so the chair, the rug, and the heater stay where they belong. A heater that passes a test afternoon is far less trouble than one you discover is broken in the middle of a cold night.