I have some problems with the previous answers, so I'm adding mine.

"Job hopping" is a concept that's hostile to the employee. And whereas there are certainly people who quit too often, when the first, smallest problem appears, for every such a person there are 10 employers that advertise jobs in data entry as data science jobs and that lie to you that this bonus that according to your contract makes out 20% of your salary and is dependent exclusively on the company's performance is normally paid, although it hasn't been paid for years. They all profit from the concept of job-hopping.

The OP's situation - not doing anything, being idle - happens a lot when someone starts a new job. However, it shouldn't last longer than a few days, a few weeks maximum. Assuming the OP is still quite young, it's important for him to learn and gather valuable skills. He won't gather them by sitting and waiting. Not to mention, a bore-out is as awful to go through as a burn-out.

A cautionary tale: I joined my current company a bit more than a year ago. After a few weeks, it became clear that there was no job for me since my team wasn't responsible for what I was promised in the job ad and interviews at all. And the culture was toxic too. So I started to search for a new position... And was told by companies that I wanted to give up too early. It was insinuated it was to be job-hopping.

Now after a bit more than a year, the situation didn't change a bit, it actually got worse. I've wasted a year of my life not learning anything, sitting and waiting and trying to cope with my bosses being aggressive. Fortunately, I've found a new job now, but even now my one-year stay at the current job is seen as "job-hopping" by most employers.

The concept of job-hopping makes it possible for employers to lie in the job ads and to treat employees like dirt.

The OP should give it a month or two. If nothing changes he should go to the boss and explain his reasons to leave. He should have no bad conscience.

The only problem here is he wants to go back to the previous employer. There were reasons why he left it and whereas an even worse employer makes him think positively about the past, he should take a closer look at whether going back is a good decision, It's like missing your ex when your new relationship results problematic. It might be a good idea to go back, but it normally isn't.