T.,
I have already prayed for him, now here is my advise.
Because your son is only 17mo I would do as suggested and take him to a dr if you see it spreading in redness or if he has any other symptoms. Read online about brown recluse spider bites for a while and you will soon realize that it is nothing to mess around with.
Since it formed a blister and burst I would put neosporin (triple antibiotic ointment) on it until you get him to a dr.
It may not be a brown recluse and you may just need to treat it like you are trying to keep down infection and treat against an allergic reaction. If your son can take Benadryl by mouth use the recommended dose every 4 hours to reduce the histamine affect in his body. This will make him sleepy most likely, so don't be alarmed about that. (I can't even take it without fighting to stay awake.)
However, I still recommend that you get him to a dr if after 24 hrs he is not any better. (drawing the ring around the redness to see if it shrinks or grows is a good idea)
Please post and let us know how he is doing soon.
~S.
PS
If you see anything that looks dark or black in the center of this bite, you have no choice but to take him immediately to a dr. or emergency room.
It has always been my experience that to go to the dr office in the day is better than waiting till the e.r. is the only choice at night. There are a lot more germs that he (and you) would be exposed to in the e.r. Then there is the waiting that is no fun for a sick child.
Just a thought.