baby gasps and chokes when on his back

M

hi after my baby feeds (breast) I hold him up for about 20+ minutes and burp him and whenever I put him down on his back he seems to choke and spit up milk. He only feeds on one breast and doesn't feed longer than 10 minutes ( he is gaining weight so no concern that is isn't drinking enough) but be does spit out on most feedings. He gasps when he is on his back and seems to have to catch his breath. any advice ??