I am pregnant with #3, honestly I wish we would have decided on #3 sooner. My girls will be 3 and almost 5 when this baby is born. I liked having my girls really close together (21 months apart). I think you could start trying now, will your son be able to go to kindergarten next year? If you got pregnant now he would be starting about the time you deliver give or take a few months.
The way his birthday falls, the beginning of September, he will not be able to start kindergarten next year. He will have one more year of preschool. And that's only 3 days a week. DS 1 and 2 are 2,5 years apart.