D.,
I would steer away from the formula - you are having no problems pumping, and he is having no problems eating!
Chances are, your little boy is like mine - he just needs a gentle burping, and to sit 45 degrees for a little while. Either a carseat or swing, or even an exersaucer (if he's old enough? not sure how old he is?)
Enjoy being able to give him comfort for the little while you will be able to. :)
One aspect of feeding my little boy was watching what I eat. I eliminated cruciferous - broccoli, cauliflower, asparagus, artichoke, etc. (there's more), nuts (especially peanuts and walnuts), chocolate (for a little while none, now I can have it once in a while), and garlic and onions (includes chives, green onions, etc) all powders, dried or anything.
I can give him some of these now that he's able to handle the gas better (it's not leaving him with the screaming meme's in the middle of the night now).
When I had an oops or just had to have something with flavor, I gave him Mylicon before I fed him after my meal. That seemed to help a lot too. I don't do it often, though.
But, oh, don't start formula until you've gone a few other avenues at least. Formula is just as likely, if not more, to give him gas. Even the gas formula isn't fool-proof.
Find a few new songs to sing, and you'll have more fun rocking him.
You're lucky you can get him on your shoulder - I'd wind up with a shoulder of spit up (or worse, he'd overshoot and get the carpet, bed, chair, or all of the above).
Good luck! It's worth the work and the wait!
M.