Pocket sized heated survival shelter - Wind Break, Insulation, Heating - I carry and use these things.

Outside he has Minus 4*F = Minus 12*C.
Inside he has 41*F = 14*C

That is an increase of 44*F or 26*C.

IF he was able to get into a better more shelted position, against the wind, and he could SEAL the tent up a whole heap more, I think he could get a temperature increase up to 70*F or 22*C.

Add in some extra insulation inside the mylar sheeting. like some quilted polyester lined cotton blanket... and he could easily get 30*C or 86*F inside that space.

And possibly drop the tea candles down to 1 or 2 - for an extended heating.

https://www.spotlightstores.co....m/sewing-fabrics/fab

2.5 meters of these - upper and lower in a single sheet.
Inside a water / wind proof layer = toasty warm and doesn't soak up moisture.

2 or more layers in a very cold place - Down to 0*C and very strong stiff cold winds are in my territory.

https://www.spotlightstores.co....m/search?q=polar%20f