Male bettas vary in temperament; some will tolerate tankmates others not at all so it's unwise to add tank mates unless you have a second tank to move him to if he doesn't take to any new additions.
Your Biorb seems to be missing the ceramic media from the bottom of the tank. This is a major part of the filtration system so needs to be in there. The sponge in the middle section may not be enough to deal with the fish's waste output and certainly couldn't cope with more than one fish.
I do have the ceramic media in this tank but due to the bowl shape the photo makes it hard to see.
The tank has been up and running for around 4 months but due to myself moving to Asia for 2 months I had to donate the previous fish to a family member but asked my brother to keep up with fishless cycles whilst I was away so it would be ready for when I came back (2 weeks ago).
I dont currently have a second tank set up so I wont risk adding anything with him.