Agree with the others, would tend not to lance due to infection risk unless you are experienced at this and can get him onto something like concrete to prevent infection.
If it's blood trauma (a "bash" for instance) then as others say it should go down soon, and the fact that it is still growing after a week might indicate against this as a cause.
If still growing, more possible would be a puncture that has got infected - a thorn can be sufficient.
If it persists and continues to grow would consider getting the the vet out - antibiotics would be indicated with possibly the vet lancing - if he does can you get the pig onto/into something where he won't be lying with it against mud? - a shed etc.?
If it stays the same size, and is causing no problems, then as BB says might leave it and see what nature does.
As Mandy sensibly says only you can be the judge in the end - us soothsayers haven't seen it