K.C.
When his bladder is more mature (i.e. ready for potty training completion) then he will be able to hold his urine for longer periods of time. If you leave the house in underpants, just be sure to bring some extra pants (and socks too)! I started with just short trips with my son to help build up his (and my) confidence in his ability to stay dry. Good luck!
PS. I'm not a big fan of ditching the diapers personally because unless you like to change sheets a lot and wash the cover for the car seat, life can get pretty messy.