Believe your instincts!! Same things - arching his back, gassy, hiccups, burpy, and he spitup all the time even hours after being fed. Best thing for acid reflux is to keep him upright as much as possible - while eating, sleeping, playing. My second baby was a reflux baby and spent a lot of his life either in the sling, car seat, and swing. I also had to lay him tummy down cause he spit up sooo much (we co-slept mostly), once we were napping when he was about 6 weeks old and he was sleeping on my chest and spit up so much that i had to change him, me, my pillow and the bed! He eventually outgrew it after 6 months when he could sit upright on his own.
Ditto then suggestion to talk to the pediatrician about it. My little guy gained weight, but not as much as he "should" have - barely a couple ounces verses a half pound over a month's time. And while we opted not do medicate right away (I was a SAHM and breastfeeding so could make some physical changes to help mine and medication was only suggested if he was loosing weight) , you might want to consider it if only to keep your son from hurting.