Yes, there is some form of separation anxiety that your pittie is going through. It is actually mild at the moment, but if it isn’t nipped in the bud soon, it could really start to get out of control. And yes, it can get much worse.
To be honest with you, crating him would be the best idea while you are gone. You could fold the crate up when you are home, so that it isn’t in the way. Space can be a limiting factor, but you need to get Riley’s issues under control first. If you do not want to do the crate thing, what about getting a baby gate and confining him to one room?
Crating will also help with the housetraining. Most dogs do not want to potty where they sleep, hence the crate. Plus, it is always good to have a dog crate trained, even if you quit using it. A dog needs to know that a crate is a safe place, especially if you need to evacuate your home for any reason. We don’t use a crate for our dog anymore, as she has never been destructive or troublesome in the house. But, we do put it together for a week or so, and periodically make sure that she still remembers the crate command and that she still enjoys it.
You have a 6 month old puppy. A puppy is going to do things that will drive you bonkers. But, it is all about management. Do you make a big ordeal when you leave? If you do, tone it down. The best thing to do is exit uneventfully and come home without any fanfare.
Provide Riley with things to do when you leave. Freeze stuff inside of a Kong or two. That will take him some time to get through being that he is just a pup. Be careful of what you give him to do while you are gone! Bones and things like that should be monitored at all times to avoid choking, etc. Please do not give them to your dog when you are not at home. I think it would be better to come home to what you are coming home to, then a dog who has a bone fragment stuck in his throat and is dying.
There are many puzzle toys out there on the market that a pup would really love. Treat/food balls are a fun way to keep a dog occupied. Give your dog his breakfast in a treatball when you leave. If you leave late in the morning, give him half his breakfast in his bowl, then the other hlaf in the ball when you leave.
If you don’t want to crate him or confine him to one room, place treats throughout your apartment where he can find them. That will give him time to look around, use his nose, and play the “find it” game. That is also fun to do when you are home with him.
When you leave for the day, give him a few treats before you go and then just walk out. That will cause him to think that it is good that you leave because he gets treats! We still do that with our dog. She gets excited to see us leave.
Make sure that you potty him frequently before you leave. That way his bladder and colon will be empty. Make sure that you praise him when you are outside with him right after he potties so that he knows that is where you want him to go. Some puppies take longer to housetrain than others, and sometimes you just have to start from the beginning again.
Best of luck to you. Re-work how your days go, vary the way you leave, and vary the things for Riley to do. In time, he will outgrow this and what you are going through now will be a distant memory.