I agree with Chris. You may have already made the decision, but just to reinforce that you do want 12 hrs between shots unless you've got someone with experience to look at his spreadsheet and let you know that it's ok to shoot early. Because Lantus is a depot insulin, meaning it builds up in the body and slow-releases, if you shoot early it acts like a dose increase. You're increasing the overlap between the end of one shot's cycle and the beginning of the next shot's onset of action.
Everybody has a "fur shot" once in a while where the whole dose doesn't get in for some reason - sometimes the needle pokes through the skin and the insulin goes outside the body, or the needle bends, or the kitty moves when you're shooting and the needle comes out. When that happens, you never reshoot, and you just know that the kitty might be high for a little while until they recover from the fur shot. You can count skipping the shot tonight as a furshot - he'll recover from it. Just skip the shot tonight and shoot when you want to in the morning. He'll be ok.