Nothing is recommended for keeping with bettas.
They are called fighting fish for a reason, and as docile as they may seem, can literally snap months down the line and kill everything or themselves.
That being said, he's in a pretty big tank all on his own (somewhere around 16g for the US folks).
Common tank mates are snails (nerite or mystery), and are the least likely to bother each other.
Cherry/various shrimp are often tried, and either work, or become expensive betta chow.
Cory are quite often thrown in, and typically work as they don't invade the bettas space, and they don't have the bright colours or flashy fins to torment the betta. However, they should be kept in groups of at least six, and that may be pushing it unless your tank is long/wide/low.
Neons are often put in for some strange reason. This is not recommended as they prefer much cooler water than a betta. If you must go with bright blue and red fish, cardinals are a better option.
Basically, anything smaller (but not small enough to be eaten), fairly dull colouring and short fins... may work out.
Just remember that just cause your betta's the most chill dude on earth right now, doesn't mean he will be forever.
Personally I'd look into something like threadfin rainbow, or normans lampeye. I have no idea if they're condition-compatible so you'd need to look into it if that's the route you wanted to take.