Thank you both for the feedback!
I have a slight feeling that the reason he started going next to the litter box in the first place is because I wasn't cleaning it enough. And since I left the cardboard there and did not clean the area, he assumes it is perfectly fine to do so. I'm beginning to think that the proximity of the other boxes doesn't bother him since he continues to urinate and defecate in the same area. Furthermore, he may despise the cheap meijer litter that I purchased to act as a filler for the boxes.
Sounds like I need to throw out the cardboard, bleach the area, nature's miracle the area, and provide some clean litter boxes that are a bit further apart than they are now.
I'll check back with this thread next week to let you know the progress! Thanks again for all the advice!