If possible, I would continue to pump and give him breastmilk in the bottle. But I know that for some Moms and situations that is not possible or what is best for the family lifestyle. So you need to take into account your child's health, and your family's lifestyle and then make a decision that benefits everyone. Don't feel pressured to breastfeed or formula feed unless you know it's what is best for you and your child.
If you decide to continue with the formula, I would try another formula. Ask your pediatrician if he/she has any samples on hand, so you don't have to buy the formula until you find one that your little one likes/can handle digestively.
What kind of tummy issues does your son have? Both my kids were lactose intolerant for the first year of life. My son drank Similac Soy until 12 months and then switched over the Nestle Good Start regular with no problems. He is adopted and I tried to breastfeed him (yes it is possible to do this), but for medical reasons on my part, it was safer for him if I stopped. I was on a medication where they weren't sure if it passes through breastmilk or not, so we decided to take it safe and not continue breastfeeding. With my daughter (surprise bio baby), I breastfed her solely for 3 months. Then I had many clogged ducts which caused sever pain and they had to be lanced out by my OB twice and after each lancing it took about 4 weeks to heal so I could feed her without any stabbing pain. Because of that, my milk started to dwindle and by 6 months I was completely dry. So at 3 months I started feeding my daughter formula every other bottle. And by 6 months she was completely formula fed. I had planned to breastfeed both my kids for as long as I could, but health and other things prevented that. We tried about 3 formulas with my daughter before we found the one that worked for her (Similac Sensitive also known as Lactose Free). With both my kids, when they tried to formulas with lactose in it, they would be very gas-ey and fussy. So if your son is having those kind of symptoms, I would suggest trying the lactose free ones first, then the soy (some kids are allergic to soy, so our ped said to do the LF with my daughter first).
Thankfully our pediatrician had TONS of sample size formula cans so I didn't have to buy any formula until we found the one that worked for my kids. Those sample sizes cost about $15 a can at the store, so it doesn't hurt to ask your ped. You can also ask if they have some coupons/formula checks on hand too. The manufacturers give the formula and samples to peds and OB's (so ask your OB too) to help market their products.
OT: Try getting onto a Mommy board like www.mommysavers.com or even the one offered here to trade or receive formula checks/coupons for free. Another site would be www.kizoodle.com. There are lots of sites out there where you can trade or buy items from other Moms for little to no cost. And definitely sign up with the formula manufacturers website to get coupons and check directly from them.
Sorry this is so long!
Good luck. I hope you find the right formula for your little guy.