Perfect timing for this topic, as Jos. A. Bank has a buy one, get two free sale today.
There are a few options when you need a summer suit. Cotton, linen, poplin (a cotton-poly blend), seersucker, and summer wool. JAB has several of each. J. Crew also has some excellent, probably higher-quality and younger-person-fitting cotton suits as well, but I haven't gotten a chance to feel the fabric in the store.
Seersucker, a cotton fabric which derives its lumpy texture from the unique weaving process employed to make it, carries a certain Virginny panache in this part of the country, so wear it with confidence and simple colors. Seersucker plus knit tie equals panache. The narrow stripes also tend to mask sweat a bit.
Poplin is your basic summer fabric. I understand you can wash-and-wear it since there aren't any fussy wool fibers to worry about. That said, I wouldn't necessarily recommend washing and wearing. Also, it feels heavier to me than the other fabrics, both in terms of mass and breathability.
Cotton and linen wrinkle, especially linen, but that is part of the charm.
Some day, I'll devote a post each to these lovely fabrics, but right now, I just don't have much expertise. I don't follow my own advice like I should, but I don't have to, because I move from air-conditioned bubble to air-conditioned bubble all the livelong day! I could wear bloody winter tweeds and still be comfy! Ahh, the suburbs.