What size tank is Cal in right now? Does he have a heater? What temperature is his tank at (remember that just because the ambient room temperature is in the 70s doesn't mean that'll be your water temperature...my house stays at about 68F, but my smaller tanks dip down into the low 60s at night...luckily there's nothing in them anymore)?
The minimum tank size a betta should be in is 2.5gal. I don't usually recommend anything less than a 5gal for a single betta. They like space to swim. The tank should be filtered and heated at a constant temperature in the high 70s (my boys like it about 79F). The plants should be either live or silk as plastic plants can tear a betta's fins. All decorations and plants should be able to pass the "stocking" test. If you can run a nylon over the item and it doesn't snag, it's safe for a betta...if it does snag, it's not safe.
As for tankmates...that REALLY depends on your betta. Some males are fine with tankmates. I have my son's betta in a 29gal community tank with five platies, four male guppies, two otocinclus catfish and an upside down catfish. He doesn't bother anyone and they don't bother him. However, I have two other males (a crowntail and a feathertail) that I wouldn't dream of putting into a community tank because they're too aggressive. In fact, my feathertail now has a tank all to himself because he was trying to attacking his two neighbors in the 10gal divided tank he was in.
IF you think he'll work with tankmates (I usually go by if they flare at their reflection or other males...if they do, they get to be on their own...if they don't, they're community worthy), I would only do a couple of platies or else it'll get over-crowded. DEFINITELY put a heater on that tank. My platies are much more active at 79F in the 29gal than they were in their old tank at 73F. Add the platies to the new tank first (AFTER IT'S FULLY CYCLED), that way when you add the male betta, he won't automatically go claiming the tank as HIS territory and trying to drive off any intruders (the platies) because they were there first.