I think you should do the 100 instead of the 40l ^.^ I mean, if it seems too deep for your guy he'll just stay closer to the top. I know one member has a 125 or something with multiple males, a sorority, and some community fish!
Yeah, it would be like living in a mansion Lol. Going from a cup to a 100g tank... that would make him just so happy. I personally would go to the LFS and choose a couple more males, the calmest ones, and add them if you go with the 100g ^.^ Sorry, it would just look awesome, and I'm too addicted to Bettas.
But yeah, if anyone came to your house and saw a 100gallon tank with a Betta in it... might just make them never use a bowl again.
It would be pretty neat to be able to keep males together. I am just not sure if I am experienced enough for that yet. Plus if something happened I don't know what I would do! What if I combined my 20g sorority with the 20g community? It would be nine girls and one boy. Does that still seem dangerous? There would also be guppies, rasboras, cories, and a pleco. Plus shrimp and snails lol.