Maybe instead of getting him a 5, you can give him even more space by adding him to one of your exsisting tanks? Bettas can do great in communities..depending on what they are stocked with already, of course.
Otherwise if a 1.5 is all you have for him, then that is all you can do I guess. You'll need a weak heater to heat something that small. The Tetra they sell at Walmart for 2-15
gal tanks works good in tanks that small. I've had probs with these heaters being defective out of the box and have had to exchange..but they are the only I have tried that will work in a tank that size. Make sure you use some cycled filter media from one of your other tanks in the 1.5's filter so it's instantly cycled.