If he didn't cough or sneeze, your only concern would be him simply breathing out. (If he was even sick, that is. Also, there is a chance that your 'moron without a mask' could be someone who has already had Covid-19 and gotten over it, and is therefore immune for a while.)
Anyway, breathing out the virus could shed a few viruses into the air. This is not like sneezing or coughing in terms of how many, forcefully blown across the room, moist, or large the particles might be. The chance of getting sick and the level of illness a person gets is directly related to how much exposure they have had. You being in line in front of someone for a short time who wasn't coughing or sneezing is not much exposure.
Some people won't wear a mask. Stores don't have bouncers who will throw people out for that reason. All you can do is either assume the risk and go about your (modified) life, or stay in all the time. If your city is close to entering the green phase, your risk is not like a place where the case numbers are rocketing up. While it's annoying when other people don't follow the rules, you wearing your own mask will help you feel more in control. (My dentist even wears her face visor at the grocery store. Everyone has their own comfort level.)
Please take care.
There's probably no risk from this incident. You don't say how old you are or whether you have co-factors that make covid more dangerous for you, but the person was behind you, not breathing right at you, and the contact was quite brief. But you do have the right to tell someone like that to please exercise proper social distancing. Be polite about it but do insist.
I had a similar situation at the hospital. I went in they checked my temp an asked me questions. They were all wearing a mask except the two receptionists. They made me take my gloves off and both didn't have a mask. Scary but I was wearing a mask. So far so good. So you must be feeling okay now?