Starting to combo feed but struggling with formula

My LO is 4 months old. I’ve been exclusively pumping the entire time. I’m early December we tried to introduce formula since I’m going back to work soon and want to make sure we have enough food for him. We had samples of Enfamil Neuropro and Gentlease as well as Kendamil Classic. He refused the enfamil. He would actually tolerate Kendamil. We started giving him one bottle of Kendamil a day about 3 weeks ago. Just recently though he started getting extremely fussy. Yesterday we trialed only breastmilk and he did fine. However, the day before that, he got super constipated and cried for 4 hours straight. The pediatrician told us to give him prune juice and that worked. Today I made up all his bottles and put some formula/breast milk in only a couple of them. He’s fussing again.

We’ve tried gas drops and belly exercises. Is there another formula that you’ve had success with in these instances? I’m worried he won’t like any other brand but I’m not sure if I should do a different form of the Kendamil or if I should just stick it out with this one.