As @jaytkay says, you absolutely have to dress for the conditions. For a mechanic in an unheated garage that’s going to mean good thermal underwear, maybe even silks under that for the way they can wick moisture off the body and into the outer layers of clothing. Quilted flannel shirts, heavy pants and his (inevitable) Carrhart (or equivalent) coveralls.
A few important points about staying warm in the cold:
1. Keep hands and feet dry and as warm as possible. If the rest of you is warm, but your hands and feet are wet and cold, then you’ll be miserable. If I can keep my hands and feet warm and dry, then I can “take” a lot more cold in my midsection. One thing that can help a lot in this regard is to wash hands frequently in warm water, and run that water over the insides of the wrists, too. That’ll warm your whole body, or feel that way. (It helps in reverse, too, when you need to get cool quick.)
2. It’s important to dress in clean and dry clothes. Clothes that get damp from sweat, or greasy from work conditions, are not going to be effective at keeping him warm.