Black and white border collie mix with blue tag

I lost my black and white border collie mix, Scout, yesterday around 4pm near Market Square in downtown Portsmouth, NH. He's about 45 pounds, medium build, with a lime-green collar and a blue reflective tag that says 'Scout.' He has a distinctive white blaze on his chest and a small nick in his left ear. He’s generally friendly but may be wary of strangers, and he recently had vaccines but is up to date; he’s microchipped (number ending 1234). I really need him back—this little guy is my best buddy and this has sentimental value; I worry about him wandering off near the river.