MAYBE he doesn't like the taste of his kibble very much that's why he eats just a little. But the next day he feels very hungry so he has no choice but to eat a lot.
Just a wild guess
But the problem when dogs don't eat the right amount of food at the right time or schedule is that they can become acidic.
There was a time my puppy hasn't eaten in 9hrs. she threw up a yellowish fluid. So now I feed her 4 times a day, every 6 hrs.
Btw, I do not free-feed, I noticed when there's an abundance of food in her bowl she loses interest in eating. But when there's just an ample amount she eats like crazy and cleans out her bowl.
I also do not give treats very often or regularly and when I do I just give a tiny piece because she'd prefer to eat treats more than her kibble.