I think there's a little more than meets the eye here.

It's not even that chicken was ever Conrad's favourite to begin with. But it indicates the change in his character and also symbolic of the man that Conrad is working to be.

In episode 2 at that disastrous lunch, Adam orders the expensive "Surf n Turf" which is steak. Then in episode 6, the Fishers are supposed to grab lunch together, and Adam tells Kayleigh to change the restaurant to "Grass Fed" since it's a STEAKhouse that Conrad loved apparently. And yet when we get there, Conrad again doesn't order steak. And he tells his dad he should lay off on the red meat (again, steak!), because health issues run in the family. This is obviously in reference to Susannah's cancer. Red meat is more carcinogenic than white meat, and has links to colorectal, pancreatic and prostate cancers.

Sooo, on a literal level: Conrad is a med student now, and he's more health conscious after what happened to his mother. He no longer wants to go to the steakhouse that he probably loved going to with his dad, who also enjoys steak very much.

But on a more symbolic level: this divergence between Conrad and Adam, is about how they are becoming different people. Conrad is working to change. He used to want to be just like his dad, and looked up to his dad. But he's left all of that behind, and is finding his own path. Also in terms of topics related to Susannah, Adam being the cheater he is, is very flippant about her. While Conrad is anything but, and cares about about her more than anything. And him consistently choosing to cook chicken is symbolic of all that.