Throwing up and gagging after nebuluzer

My son has bronchiolitis and has been on nebulizer treatments since yesterday. He has been vomiting and gagging on mucus after two treatments today - only immediately after. Doctor says this means is working and to not feed him immediately before. It's the mucus coming up that scares me. I'm so scared he's not going to clear it when it happens. I'm also frightened he's going to go into respiratory distress. I hate that I can't relax. I'm constantly waiting for the worst to happen.