My son wore a little blue checked jon jon from Strausburg babies (I got it on ebay:)) with a white onsie underneath. Super cute.
His normal summer wear were tshirts and these athletic lightweight shorts from Osh Kosh. They are that kind of windbreaker material that gets wet and dries real fast. It seemed my son was always jumping in a fountain or sprinkler somewhere, so they worked out great. I bought several pairs of shorts and tshirts from oshkosh, and the colors all worked together and so they were easy to mix and match.
I'm also a Gymboree Junkie, but am too poor to buy full price. I haunt the sale racks and buy off season for the next season. The clothes are almost always super good quality, and really cute. If you buy off season, you end up paying about what you would pay at Target or less.