Hi S.,
Before switching formulas, I too would recommend trying the infant gas drops if you haven't already. Some babies are just naturually gassy, my own pediatrician has tried every formula for her son and she says nothing has helped him either. My son is on Similac Advance and experienced a lot of gas around your son's age, but its tapered off considerablly now that he is 18 weeks. I use infant gas drops pretty regularly in the evening because they work quickly, especially after his last bottle and before he goes to bed. I have used Mylicon, but its pretty expensive, so I switched to the generic brand they sell at Walgreens and it works just as well, for about 1/2 the cost. Its at least worth a try before switching formulas that may risk increasing the constipation and spit up again.
Good luck!