New cat has mushy stool

My new 1yo cat has been passing loose, mushy stool. It’s not liquid, but not firm. His foster had him on an all-kibble diet, so when he got here 2 weeks ago he was basically constipated.

Now he gets 2 cans of fancy feast a day with 3 tbsp of kibble as a snack. As a male cat, I’m trying to make sure he gets plenty of water…so I do add 2 tbsp of water to the kibble.

I’m also having fun switching up various meats and flavors. He’s excited to eat anything!!

At what point should I be concerned that his stool isn’t firmer, and what could be causing this? He is otherwise energetic and doesn’t seem ill.