One of the fist things people should know about bettas is their aggressive behaviour. Betta fish are nutorious for their fighting, hence the name Siamese Fighting Fish. Males definetly cannot be kept together, and males and females should only be kept together for the brief period of spawning. You should separate them as soon as possible, if you have not already done so.
The male builds a bubble nest when he feels ready to spawn, which is pretty much the whole year. Mine builds his bubble nest just along the edge where the bowl meets the water. If you want to encourage your betta to spawn, you can place a clean leaf, a styrofoam cup cut in half and placed on the water, the cut part on the water and making a dead end tunnel (tapes to the edge of the glass), and he should build his bubble nest there. However, mine builds one without any encouragement and I was slightly hesitant to clean his bowl the first few weeks. XD
If you want to stimulate him to build his bubble nest, I heard you can raise the temperature of the water or keep the female in sight (though not in the same tank/bowl as him)
If you are going to (or looking into) breeding him, and the betta you have are veiltails, do not mention it at this forum as you will feel uncomfortable reading the replies. Many people highly discourage the breeding of veiltails because it points to 'betta in a cup' and many homeless veiltails. If you wish to breed, many will insist for you to purchase the more expensive but better bettas, for their genes and looks, and they're easier to sell or give away once they have grown because of their prettiness. I have four veiltails, and mentioned breeding, and I can assure you that you would do well to not to do so - maybe at aquaticcommunity.com There are more 'noobs' there.
I am one.
I hope this helped....and I hope you take the advice if you were considering breeding. If you weren't even considering breeding, and just wondered about bubble nests... well, there's 10 minutes of my life that I will never get back.
It's all good, though~