Please don't be cross. Or threaten bedding and mattress covers from Santa, that's cruel.
He is not doing it on purpose.
He has just started reception, that is when my Dd2 started to wet the bed, and she had been dry for 2.5 years before that. She's in year 1 now.
I would make him drink loads after school and then only a little after dinner, give him time to get rid of it before bed. One last loo trip before bed is essential.
Talk to him about school, what worries him, talk to the teacher without him, if you haven't already done so.
It may be just the change, and there is nothing wrong, just adjustment.
My Dd still wets the bed, especially after half term holidays and sometimes we have a wet bed Monday mornings. We haven't quite got to the bottom of what's going on apart from the fact that she's rather stay at home with me.
I'm not happy with the extra washing but change bed/shower DD without comment. She feels humiliated anyway. Please, whatever you do, don't be cross. You'll make the problem worse and damage your relationship.