Is there a way to build ones' tolerance to cold temperatures?

Don’t get older. Seriously, cold affects you more as you age IMO. He needs to dress warmer. Long johns. layers, etc under what I assume are coveralls? I don’t know any way to build tolerance.

Get fat, the fat insulates you. That is the only proven way that comes to mind.

I grew up in Michigan and live in Chicago.

I believe the axiom “There is no bad weather. Just bad clothes.”

The key is layers. Multiple pairs of socks. Sweaters over shirts over undershirts.

Wool is THE BEST.

You can find lots of cheap wool clothing at Sierra Trading Post

Cold intolerance could be caused by some medical conditions, so he might want to discuss it with a physician.

Does the garage not have any space heaters available? He may want to check with OSHA (if in the USA, or with government agencies where you live) to find out if there are regulations for work area temperatures. My ex and now my son are mechanics, but the garages have been heated (although not very efficiently, since that’s the nature of garages that require ventilation).

Meanwhile, dressing in layers is the bast way to handle being in colder climates for long periods of time. WinterSilks.com has some lovely silk long underwear that feel nice against the skin but provides insulation without being itchy or sweaty – of course you pay more for silk. Athletic stores, such as Eastern Mountain Sports and REI have lots of undergarment options, as well. Land’s End and LL Beans are other options for finding good layering clothes.

I’ve had similar jobs in the same environment. It can be miserable. Layering of clothes helps, but there’s a limit one can wear and still move effectively. For me, keeping my head warm with a stocking cap helped. As did changing my shoes and socks if they got wet. These days they have some nifty, inexpensive chemical hand warmers available. They help me a lot when placed in my socks or hat.

Wrenching isn’t always ‘active’. Taking 5 minutes an hour to move quickly by jogging or doing jumping jacks helps get the heart rate up and the blood moving. When one is cold, it’s often easier said than done.

When I was chilled, I really appreciated 15 minutes in the warm, local diner where I would down a bowl of spicy hot onion soup. Spicy hot foods will increase metabolism.

I don’t know enough to recommend any specific herbs, but if you search for herbs that are recommended for dieting purposes, you’ll find some that are intended to increase metabolism.

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.

I second the silk longjohns @hearkat provided the link for. They are lightweight to wear, and quite warm; they make several different weights for different low temps, and they are great for layering.

Cayenne Pepper

Spend as much time in cold whether as you can… you get used to it after a while! If the pain is really that bad “in the bones” stay away from the cold or dress for the weather more

Bit of an obscure one, but qigong or tai chi. Once you know how to circulate your qi, it can really warm you up. It takes a while to get the knack of it though….