Sometimes after drinking milk, especially at night, my son does not burp even after i rub his back for at least a few minutes. if i put him back to sleep, does this mean he is uncomfortable and gassy throughout? thanks

Not necessarily: All of us including infants swallow some air with feeds.Some babies are easy burpers & some not. I wouldn't assume they are in pain if they don't burp. It would be good not to feed at bedtime or during the night as a rule. If they use that as a crutch during settling they will need it in the night to re-settle. If not fed at bedtime it avoids this. Water in the night stops the urge to wake.