If they showed up on the outside of his fur, after a walk through brushy area, I'd say it's likely that they are ticks. Here's a picture:
Female American dog tick
They can carry diseases, so you want to make sure that you get rid of them before they bite your dog. Some flea treatments will deter ticks, some won't, so check what you use for that. Also, ticks like small moist places, so make sure that you check your dogs ears, between his toes, and his privates for them. According to my mom, they are really bad out where she lives, probably because of all the rain, and the treatments that used to keep them off aren't being as effective this year as usual.