Apologies if it's already been said, as I haven't read all 5 pages of replies.
Try to tell him what you want him to do, not what you don't want him to do. Our instinct is to say "Don't pour that drink on the floor!" whereas it's much more effective to say something like "Hold your drink carefully" with lots of praise given if he manages it.
It can be really difficult to think of a positive thing like that to say on the spur of the moment as it doesn't come naturally, I needed to practice when I looked after my grandchildren but it really does help.
I realised I was trying to talk like a satnav, they say "At the next roundabout take the second exit." they don't say "Don't get off the next roundabout at the first exit because you need the second one."
At 15m he's too young to understand about the 'naughty' step or even the concept of being naughty, we all have different definitions for that anyway. If you and the parents decide to introduce it later it's better to call it a time out or calming down step/area. And at 15m he's also too young to understand any lengthy explanations of why he shouldn't do something, so keep these short and only if you really think they'll help.
He may not have much language to be able to express himself yet, so acknowledging his frustration that he wants to do something, but you don't want him to do it, will also help. Try to have some distractions to hand to divert him from something you don't want him to do.
Good tv is your friend in short bursts if you both sit and watch together; CBeebies stuff is good and (I think) has to be produced to strict guidelines - Hey Douggie is my and my granddaughter's particular favourite!
There's also a new BBC Under 5s parenting website, Tiny Happy People, that looks really good so it may be worth your while giving that a look as well.
Wishing you all the very best Cher.