Reebokthedug

Hi I’ve been fostering a 5 year old ex racing greyhound for 3 weeks. Sometimes when he is on a walk he suddenly freezes and refuses to move. Most of the time it’s when he sees other dogs but some times he does it for no apparent reason. This morning he did it for no apparent reason and refused to budge for 10 mins. Does anyone know why he’s doing this and what’s the best thing to do when he does it. Thanks

Walking