Children's fiction book about the multiverse

The book contains a multiverse which is explained as follows:

The multiverse is infinite parallel realities almost in the shape of a sphere with layers closer to the center being better (more pleasant for the inhabitants) while layers further from the center are worse. The closer two layers are together, the more similarities they have. I’m not 100% certain it was a sphere, but it was definitely directional.

The plot as best I remember it is as follows:

We are introduced to the protagonist, a young boy, who has decided not to go to a party with popular kids (maybe at his school?). The boy finds a wall built by, I believe, his grandfather. There is a special card which the boy finds and gets access to the wall which transports him one layer out of the multiverse from his own. In this reality things are similar, but slightly worse; apparently he went to the party but had a bad time. Something happens and a major conflict in the story is how the boy can get back to his own reality.

A few other points:

• I believe for some reason you can only use the wall to travel to layers worse than your home layer. The boy assumes that in the next inner layer from his own, he went to the party and had a great time.
• Another part of the story plot involves something about his grandfather living under a freeway overpass. I’m not sure if that was in a different layer or his own.

Extra Details:

I believe the setting was present day at the time of the book’s writing (my guess is US somewhere in the time frame of 1960 – 1990, though I can’t remember if the country was specified). I don’t recall any futuristic technologies, besides the wall, which I believe was located at the boy’s house either in the basement or backyard. I think the grandfather was a scientist held in low regard as eccentric or crazy. The grandfather being held is low regard may have contributed to his living under the overpass.

I read the book in the late 90s so it was written before the turn of the present century.