I expect you've heard this too, then?!
Plenty of fresh orange juice, not drink or squash but juice!!
If you are being overly hygienic and cleaning up after every dribble and frantically bleaching everything out of existence at the mere sight of dust - stop now!! At the risk of upsetting you - its a classic way that puppies react if they are not being allowed to do their business when they want to - so they eat it.
Put him back in nappies till he's ready to use the loo.
Get him a friend who does his sitting on a potty in the living room (eew! I hated being faced with that at people's houses, but there we are!). Imitation is the sincerest form of flattery etc!
Stop reacting one way or the other about it, just ignore it. It's attention-seeking, but only he knows why and what for.
Dont over-react when he finally does do something, just act as if it's normal and eventually he will too.
Put some books in the toilet area and let him sit there and read them.
Whatever you do - don't let in-laws interfere with the programme!
Well, you did ask!