I would stop giving him those things indeed (that much peanut butter isn't good for him anyway - hard to know about the liver treats, it depends on what's in them). But you're well on the way to being very well trained

And if you create a picky eater now, you'll have a picky eater for the next 10 years.
Puppy appetites go up and down. That's normal. And it's because of their growth spurts. They grow in spurts, and naturally need more food when they're in the middle of a spurt - energy is required to support that, so the pup is hungry. But then the spurt stops. And so does his appetite. It's also quite normal and healthy though! When he doesn't need as much energy to support growth, he will listen to his own body and not eat as much.
Unfortunately, what often happens during this process is that we see the pup isn't eating much - and instead of just letting him eat as required, we
worry. Then we start rolling out goodies to add to the food to try to tempt them. Well, you know, it's actually healthier for him to eat according to need - including not eating much when his body doesn't need it. So we should be cautious about trying to tempt them (ie. don't do it). If you continuously add something better to his food, he'll continue to let you. LOL - who wouldn't? Just like little kids, they figure it out VERY quickly if Mom will always roll out something better every time they refuse to eat their dinner.
My suggestion would thus be to cut back substantially on those extras. Or at least, replace them with more healthy alternatives. Then give him his meal, 20 minutes to eat it - and if he doesn't, then just pick it up and don't offer anything else (ie.
don't fill him up on treats

) until the next meal time. If he is hungry - meaning his body needs the fuel - he will learn to eat when food is offered. In the long and short term, that's a whole lot better than having a tantrum until Mom comes up with something nicer