I agree with Nancy. Feed him what he was on with the breeder and do not add any human food. He is a puppy and should be on puppy food that has the right nutrients and all for him.
If you switch foods around, and he is vomiting, you will not know if there is something going on with him or if it is the food. Best to stick with one thing for now.
Just keep an eye on him that he does not dehydrate or get hypoglycemic. His gums should be wet to touch....that will show his hydration status. If he loses mental status, that can indicate hypoglycemia.
And, yes, a playpen is wonderful! That way he cannot get into anything when you leave him unattended.
They do cry for attention. It is good for him to be alone sometimes....he will get that you are coming back. Just takes time. If you pick him up every time he whimpers, he will continue that. They are smart!
Welcome to YT!