He can take a couple minutes during the day to burp sometimes. I only burp him with him lying on my shoulder at night... during the day when I do this he is awake and often tilts back which may help him burp easier. I'll try sitting him up at night to see if that works. Last night I had him in his bouncer all night (often he's in his bouncer for an hour or so) and once he finally settled at 9pm, he went till midnight, then till 3am, and again until 6am. Not sure if that had anything to do with it, as he's been in his bouncer on many occasions...
If I put his mattress on an incline, he would slide down to the bottom. Unless the incline doesn't have to be much?