Wow, djoy, you kids will have almost the exact same spacing as my first two. I was one that lost my supply immediately upon becoming pregnant, but I don't think that is the norm. My midwife expected that I wouldn't notice my milk being gone until 20 weeks or so. My son began nursing ALL the time, and that was my first clue that my milk was gone. I also immediately got sore nipples (although not this time). Hopefully you will keep your milk throughout your whole pregnancy!
I would just watch him for signs that he's not getting enough, and if you have a way to monitor his weight it would be great (my son actually lost weight before I realized there was a problem... long story). I do have to tell you, though, that my children ADORE eachother now. They play together a lot, and keep eachother pretty entertained. There are some really good positives to the spacing that your children will be.