What you are describing is probably a seroma or pocket of serum. This serum has separated from the cellular part of blood from a previous bruise. As long as it is not painful and not enlarging you could give it a couple weeks to reabsorb. It is not something you need to worry about but just keep an eye on it. It could gravitate down the leg, this is pretty common. If it starts to enlarge or become painful let me or your vet know, also if he were to develop another one you should let someone know.