So I'm moving 15 hours away from where I'm currently living, and driving straight through with my 3 month old bearded dragon. I'm wondering when/if I should stop feeding him before the drive?
Are you transporting him or her in their viv? If so, I can't see why you'd need to do that. I'd make things as normal for your beardie as possible and take them out at rest stops to get some sunlight if it's not too cold. Also, I think people have suggested wrapping up one of those hand warmer packs in something like a sock (to keep from burning your dragon), to keep them warm on the road. Those packs last about 4 hrs per if I remember correctly.
From my experience they do pretty well on car rides. Just have some cleaning supplies handy in case he poops on the way. I usually stop at rest areas to provide some time to run around and bask if needed. That depends on the weather though. They do pretty well at normal car temps so I wouldn't worry about that too much. I really like having a carrier that clips into the seatbelt. Makes it secure during the drive but also provides a good carrying case for them. https://smile.amazon.com/gp/product/B00EW2WKZU/ They are usually OK for a while without food but since he's so young I suggest bringing a container of black soldier fly larvae. They transport well and are easy to feed on the go.