Worried about grandson

Hi , am worried about my 6 year old grandson ( as is his mum) he is a lovely friendly boy but he beginning to fall behind at school, he struggles to write down work at school although when you talk to him he has good understanding. He finds any change very hard and will often become overwhelmed and have a bit of a melt down . He wants to play all the time rather than do what he should do eg get dressed, go to the loo etc and it’s more than you would expect for a 6 year old - he gets so deep in his play and fixed that he won’t do what he should be doing . We all try with rewards, boundaries etc and all the behaviour advice but it doesn’t help him to learn to manage his impulsivity or to do what he needs to do. He is day wetting and has seen the bladder and all is ok there but he says he’s doesn’t want to stop what he is doing to go to the toilet and that is why he wets. We are following all their advice. I do notice that he appears to have a short attention span and likes things to be on his terms or he struggles. If anyone is able to offer any advice or tips that helped with their child I would be very very grateful - thank you