Do you have access to donor milk to replace part or all of the formula?
The best thing is to do all you can to increase your milk supply and give him as much fresh, raw, human milk as possible. Then donated, pasturized milk is "2nd best" if you don't have enough milk. Then fill in with formula if needed. The baby needs enough calories to grow-and if you need to use formula, then that's what you use.
Since you ARE able to give the baby some breastmilk, he's getting the antibodies which will keep him healthy. He doesn't have the "virgin gut" of a 100% breastfed baby, and therefore he won't be AS healthy as a fully breastfed baby, but he's still in much better shape than a baby who's all on formula. I don't know if it will do any good to add probiotics when he's already getting probiotics from your milk.