Second mmr

Hello, looking for some advice. Im not for a second saying the mmr caused my childs autism, we knew he was displaying signs from about 8 months old, however almost immediatly after it, he was ill and then changed completely, like a light turned off, stopped talking and was silent for nearly an entire year. He has come so far now and we just got his appointment in for his second lot of mmr and im scared incase it sets him back again. Does anyone have any advice for me, thabks in advance