excessive spit up

My son is ebf and gaining weight and healthy and happy. He spits up after every feed in large amounts. I feed him upright and keep him upright. This just started and he's almost 10 weeks. He also has a lot of saliva and sometimes when he's on his back he will look like a fountain when he spits it up. I changed his clothes 5 times yesterday. He has no symptoms of reflux. No fussiness. He's usually laughing when it happens. Idk what to do for him. He's gaining weight so the dr isn't concerned.