Grandson has red place on thigh says that it's tender he has not raised it hasn't hit anything but it's warm there are no knots and the place.?

Size? If it's less than a quarter size then you can probably watch it overnight. If he has fever or you find it worsens overnight he will need to be seen. You can mark the outline with a pen, so when you look at it later you can compare the size. It may be just a bug bite.