Just noticed that my ratty has a sore foot?

He has had this in the past but it got better overnight he doesnt seem to be weight bearing on that particular foot, does anyone know what it is, please no nasty replies as there is no need for it. I was very disappointed to read a comment from someone who seems to think the questions asked here gives them a right to mock, we love our pets and hence we look here to fellow animal lovers to respond accordingly. Thanks