Hi J.,
This sounds exactly like my two sons. One is 3 1/2 (he out grew reflux at 4 to 5 months) and one is 1 month old and we're in the midst of reflux hell! LOL. My son is gaining weight as well and formula vs. breast milk makes no difference here either. YES, spitting up formula up to 2 hours after eating makes it difficult for the little one (and you) to rest! And yes, laying down or car seat - oh J. I feel for you.
With our first son we found he was allergic to milk based formula. So we switched to soy and he was allergic to that too. So we went to Allimentum (Similac brand pre broken down formula). This didn't stop his reflux, but it helped a lot. With my second, as soon as he showed signs of discomfort on milk based formula, I went right to Allimentum and it's been better, but again not gone. I've been told that reflux happens because the valve between stomach and esophagus hasn't matured enough so fluid flows easily back and forth.
I have found that the following really helps:
1)After he eats, keep him upright for at least 30 to 45 mins.
2)When possible I put my son in the baby bjorn carrier and then he's upright for a lot longer than 45 mins. and I can get stuff done around the house.
3)**Check with your doctor before you try this** My doctor suggested giving 1/4 tsp. of Mylanta before the bottle. We have found that giving half the dose after he spits up really settles him because it neutralized the acid he's just spit up.
Good luck, and know you're not alone!