My brother has this growing in his garden and doesn't remember planting it. Any ideas?

Since no one has replied with an answer yet, I'll give my barely-useful opinion. It isn't an Aster, but I can't tell you what it is.
Could you get a photo of the foliage and another photo of an individual flower? I think that would help because I can't really tell how many petals, stamens, etc. each flower has in the current photo. Also, is your brother's garden in Texas too?

Pretty flowers! My first thought was possibly Blue Mist Flower (Conoclinium coelestinum):


or maybe:


But, my eyes aren't seeing clearly tonight so I really don't know. I agree with Danita, it will help to know where the plant is growing and if you can get a photo of the leaves or entire plant from a distance that could help narrow it down as well.

Definitely not C. coelestinum, I do know that plant. Here's a few more photos:







Yes, he gardens in DFW.

I'm thinking Gilia capitata.
There seems to be a lot of variation in the species. Your plant has thinner petals and leaves than some but I have seen photos of similar, daintier ones.
http://www.wildflower.org/gall...
http://www.wildflower.org/gall...

Globe Gilia (Gilia capitata)

Thanks, that looks like it is probably it! I'll pass it on!