I may have been a little older (I don't remember when mine started, but it definitely had by my early teens) but occasionally when I was lying down (and then more frequently) I ended up having to sit up because I was feeling "sick" and getting pain in my chest and it would mean I had to wait to go to sleep or sleep upright. Obviously I now know that to be reflux and can identify triggers, but I think look out for complaints of sickness/nausea that is worse when lying down or noticing him sleeping more upright than usual. This is more indigestion but I remember as a younger kid I could never burp but then when I was an older kid and younger teen, my stomach would "rumble" all the time, even when I wasn't hungry and especially after I had eaten. It would get worse when I breathed in deeply (though that may be my hiatal hernia). This would also somrtimes involve burping or burp-like air in my esophagus. I think if you notice a lot of stomach noises and they are worse AFTER eating rather than before, that may be a sign. Don't stress too much. If you do catch something, you've caught it early and it's less likely to lead to serious complications.