I don't see this as a grounding issue. But I DO see it as an issue of making him clean it up. Not with any sense of shame (you'd have to ease back on that if you could) but in a sense of, "this is unhygenic, it's untidy and unacceptable. Someone's got to clean it up; you did it, so you can clean it up." I would help, at least so far as showing him where the bucket and scrubbing brush is, helping him get a plastic bag which hasn't got any holes and showing him how to put his hand inside the bag to pick it up, then turning the bag inside out around it, then watching him throw it away.
it's a sort of extension of delayed toilet training in Sensory Integration Disorder (SID) kids. Sensory Integration Disorder (SID) is often a part of Autism Spectrum Disorders (ASD), we went through variations of this with both boys and to a certain extent with easy child 2/difficult child 2. She especially would get 'it' on her fingers while using toilet paper, then wipe it off her fingers right onto the walls. I wouldn't spank, ground her or otherwise punish - I made her scrub the walls down. I talked her through the steps and thanked her when she was done. We then put some nice-smelling essential oils in the bathroom (which we still do, even now - trickle a drop inside the toilet roll).
None of what we did was rewarding the behaviour, but it WAS responding with appropriate consequences.
Autism Spectrum Disorders (ASD) kids are often VERY delayed in learning normal toileting patterns. Plus we have a neighbour kid who is supposedly easy child and 8 years old, she's leaving little 'surprises' in various places in friend's houses (her parents refuse to accept this is happening - don't know what is happening at home). And I remember at that age I had a friend who used to do the same thing - I think it was about control. She tried to get me to do it too, because then it made what she did more acceptable because she wasn't the only one. With my friend, and with my young neighbour, there are problems at home beyond the parent's ability to cope, nobody's fault, but there has been a lot of stress and the child is feeling out of control. (mother dangerously ill, ambulances repeatedly called over months, child can do nothing). Not that any of that can justify it - I grew up with a mother who was repeatedly in hospital dangerously ill and I never did it (not even when my friend tried to make me).
But these things happen. Consider it akin to a toddler with a dirty nappy.
If you put shame into the picture, you may only drive the secretiveness underground. Although right out in the open like that - sounds more like he was caught short and just didn't think. In which case - he's made a mess, he can clean it up, just as if he'd been making mud pies from garden dirt, right on the driveway. Gotta clean that up too. Same attitude, but throw in the "unhygenic". And maybe offer to put him back into Pull-ups if he can't plan his time to get to the toilet.
it really does sound like he's not recognising body signals properly let, and thinking he can delay (not wanting to change tasks - typical of Autism Spectrum Disorders (ASD) also) when he really can't.
And he's not the oldest to do this. A friend from church was looking after her husband, with Alzheimers. Their house was right on the beach and he always loved that beach. She had to constantly watch to make sure he wasn't out on their back lawn (which ran down to the waterfront) stripping off his clothes because it was a nice day and he wanted to enjoy the feel of the sun on his skin.
Then one day he stripped off on the lawn again, right when she had visitors for lunch. As she was heading out with a robe to bring him inside, she found him squatting on the lawn, stark naked, following the call of nature under very public view - her guests as well as many people on the beach.
He really was a lovely, kind, gentle man, it was sad to see this happen. But what can you do? He had forgotten what was the right thing, just as I think your son is still learning.
And crazymama - the stains and getting them to wipe is another matter. A high-fibre diet keeps it less sticky, hence less staining. We have Baby Wipes and a bin available, because it makes it easier to thoroughly wipe clean, but like you we didn't have white undies for a long time. I would regularly apply the sniff test and if the undies were dirty, I'd make the kid take them off, wash themselves down there, then wash the undies out. To help remove protein-type stains, try wetting the stain with vinegar then putting in an enzyme pre-soak. You can use warm water, but not hot - hot water will 'cook' protein (think - boiling an egg). We used nappy soaker for a long time, to work on stains. It also works brilliantly (the vinegar splash etc) on sweat stains and smell.