Just let her know. We both know your pretty close to your mom and that she's chill. Plus if you really like this guy your judgement might be little blinded, your mom on the other hand will be able to point anything out for you.
Just tell her haha, what's there to hide. Being a drag racer isn't the most reputable type of racing but it's still a sport of some sort. Besides, does ur mum expect you to be dating a banker at your age. If you really care for him and he doesn likewise then tell her that. Honesty is key to a successful relationship. And that also means being honest to other people about your partner.