My 7wk old seems to be choking on drainage and gagging alot, at night or when laying on his back. help?

My 7wk old baby woke me up in the middle of the night last week gagging and gasping for air. It seemed that he had some type of thick drainage that he couldnt get up. He did this about three times in 3-4 minutes. His face began to turn purple he was gagging that bad. We were at a football game tonight and it was cool 75/80 degrees somewhere in there, rainy so we took him to the SUV and kept him in there until it stopped raining. When it stopped raining, we wrapped him in a blanket and put him in the covered stroller. He fell asleep in his carseat but when we got home my husband put him in his benett and a few minutes he did the same things as last week. He started gagging and it sounded as if he had drainage in his throat and his nose. I got the nose sucker thing out and tried to use it on his nose but didnt get anything out. So i put him in his bounce seat and he hasnt done it since. Does anybody have any suggestions? I am not a new mother either, i have three other boys. Ahh!!!