I never heard of a trial rehome either
Both of mine are rehomes and Jada was 2.5 years old when we adopted her. It took her at least a couple of weeks to settle in and be comfortable enough to eat well. It took almost a year before she would look at a toy. Bogie on the other hand was younger at 9months and settled in almost immediately.
There is no way it is just going to take a few days for him to feel like he is at home,plus, it isn't fair to him to be passed back and forth, that has to be so traumatic and stressful for the little guy.