The most likely thing happening is that your son has grown again. Little boys especially have this trouble; their bladders don't grow equal to the way the rest of them have grown. So whenever they go through another growth spurt they have to gain control of their bladders all over again. Then you add in the fact hat kids can be super sensitive and you have a child that is either too embarrassed to ask to go to the bathroom during class or too embarrassed to tell th teacher when they have had an accident. I agree that changes in routine can really upset a child, but my child is in second grade and had an accident too. When I asked the nurse about it she said that it is not uncommon for her to see children all the way up to the fourth grade still struggling off and on with that.
Be sure to give your child some understanding that he is normal and that everyone has accidents at some point. He needs the confidence to speak to his teacher so he can get cleaned up and changed and the other kids don't tease him and you can help him have that confidence by showing your support.
Another thing you might want to do is meet with the teacher and just let them know that you know it is going on and the two of you can maybe work out a system where the teacher tells your son (privately of course) that he should go to the bathroom periodically through the day.
Good luck and remember that you all are not alone!