B.C.
Having to remake a bed in the middle of the night always woke me up too thoroughly to get back to sleep, so when my daughters were little we used to double sheet the bed - 2 layers of waterproof mattress pads and sheets - then if she threw up or peed in the bed, we'd strip off a layer and dump it in the bathtub with water, then immediately be able to all get back to sleep.
If that's still too much trouble, then put him back in diapers and don't make a big deal of it. He might be acting out because of the baby -- or just sleeping too soundly to wake up and go to the bathroom.