Doubt he'll stop just be glad it wasn't crap -sometimes they crap and tend to spray things with urine .

If they never humped, or stopped for months or years after neutering, sometimes some additional testicular tissue can grow in the sacs (especially if they were neutered very very young), or inside the groin and set off enough hormones to make them hump stuff but usually there's no discharge or odor of any kind. They can have surgery for this but it's huge surgery and basically hunting for something that might not really be there so you really shouldn't go there, much too risky, expensive, painful. He might be spraying, I suppose, because neutering doesn't necessarily stop this if they were doing it before they got neutered. Or, if you recognize the smell of spraying and this is not that, his anal glands could be impacted and this can put enough pressure on their general equipment that they start humping. You can clean these out yourself; it's pretty gross and you have to repeat it several times usually until whatever's causing the impaction clears up. The vet can show you how to do this or you can probably find instructions online.

I have one cat who does this practically constantly and I think he's actually in love with me, because he'll stare at me while he does it with to the bedcovers and get quite a soulful look in his eyes, lol! He doesn't do it to anything but pillows/covers. Oh well. Usually I just ignore him but he will quit at any touch from me. Apparently it requires a certain concentration, hehehehe.