think about the logistics of it though. if they pick him up, they're gonna be in the car with this dude. do you keep a gun pointed at him the whole time? let me also point out that Carl is also with them, and Rick isn't about to take any chances that could endanger him. they're also already headed into an unknown situation, and they don't need any other variables to worry about
not to mention it could have been a trap. dude could have his buddies in the woods. that was actually a common tactic for hitchhikers back in the day. it wasn't even for trife reasons, but rather just because it was easier to get someone to pull over if they thought there was only one person (or even better, a hot girl), and then hopefully they wouldn't change their mind when they saw more people...