My ds was on Similac Alimentum for 15 months, as he had MSPI.
We went through the same process as you, first Similac Advanced, then Isomil Advanced, and ended up on the Alimentum.
It worked WONDERS for him. We also found out at about the same time that he had reflux, and he was put on Zantac as well.
I don't know where you are, but it's about $25 a can where I am, and that can didn't last very long, so it's a very expensive proposition. But in my house, it was WORTH EVERY PENNY!!!
You say your lo is fussy and gassy - have you tried the Mylecon drops? Those went in every bottle we gave ds. The medication passes right through the system, so there are no side effects.
I'd also check with your ped before switching - they may be able to test for milk or soy protein allergies.
HTH!