A good lawn fertilizer to build up thicker turf would be a good bet. Make sure is has a good percentage of slow release nitrogen, (at least 12 percent). Scotts is the most common retail brand. Apply again after about 4 more weeks. Winterizer would work OK also. You can probably omit weed control until spring; frost and mowing will kill a lot of the summer weeds...however...if weed control is needed, wait until new grass has been mowed twice.
Fungus growth or mold is not good. Pythium can take out large areas of new seed...especially if you have standing water or high temperatures.