I would suspect home is not doing their part to curb this attention grtting behavior.

If you truly feel it is done on purpose and not due to some medical condition I would either terminate or as a last ditch attempt make it not fun for him. While I wouldnt do this for the average child struggling with toileting it might work to treat him like a baby. Tell him until he can use the bathroom like a big boy he does not get to play with big kid toys, eat big kid food and he needs to live his day like the babies do.

Put him back in diapers and tell him until he is using the toilet to pee and poop he just doesnt get the luxuries of being a big kid anymore. Use this time to do a lot of cool activities with toys with little parts and lots of fun art projects that he has to sit and watch while playing with baby toys.