Spitting up

My 6 week old is EBF and he has been spitting up a lot lately. Sometimes it's right after he eats and sometimes it's not until an hour later. It's not a problem during the day and he's gaining weight fine, but at night after I feed him I hold him upright for 15-20 mins after he finishes eating and he still manages to spit up and wake himself in the process any suggestions?