It’s hard – I searched for ages, made much harder by v inconsistent sizing advice.
Decathlon do some fingerless gloves in small sizes. This was good because you can try them on and they weren’t expensive. My boy had some of these from age 3 IIRC, they were good. But obviously fingerless is not ideal if you’re buying them for warmth.
There are some kid-focussed Moto-X sites that purport to do kids gloves in really small sizes, but I never pulled the trigger with those so I don’t know about quality. Ditto Fox do some youth sizes but I couldn’t work out whether they’d fit and they were kind of expensive so I didn’t try them.
Now he’s 5, he has some of these in the 5-7 size: https://www.ebay.co.uk/itm/124623231749 NB They do a small 2-4 size which might work. The gloves themselves seem decent enough quality, too.
The other option – if it’s just for a Shotgun seat I’d probably put him in some normal warm gloves. He doesn’t really need to grip the bars, does he?