Great video as always.
I'd be willing to bet that was either an abscess or an umbilical hernia rather than a tumor.
In one of your shots it looks like it's down on his belly just in front of his penis. Is that correct?
It's not at all uncommon to see herias on boars as they can commonly get their stomach muscles torn or even puncture fighting.
when in doubt open it up carefully. If it's full of puss forget it, otherwise it's not likely to affect meat quality.
We've killed a lot of boars that had really nasty abscesses from old bullets buried in the shield or from getting tusked.