What to do if my son is getting braces so my ex-wife paid the dentist in full to receive a 5% discount and the child support court says I have 30 days to pay my half, which is $1500?

The courts usually require the parents to split actual costs of any medical or dental care.  The emphasis here is actual expenses.  If the total bill was $3000, but she received a 5% discount, then you are only on the hook for $1425, half of the amount after the discount.  As far as the time frame, most court will impose a time limit of 30 days.  So the request for payment in  30 days is also-- not unusual.  However, if you simply cannot come up with the whole amount in 30 days, see if your ex- will work with you to spread it out over a couple of months.  She could take you to court to enforce payment of the amount, but enforcement could be slightly counterproductive in that she'll have to put that amount out in attorney's fees. 

If she simply won't work with you, still pay what you can.  If she does take you to court for an enforcement action, you can show the judge that you were making payments in good faith.  This will help persuade the judge to avoid any punitive sanctions... because you were trying your best to pay your share.

If the 30 day timeline is unreasonable for you under your set of finances and you are concerned about getting stung again by this provision in your decree, you may want to considering filing for a modification which grants you either more advance notice of the procedure (so you can start saving) or a longer time period in which to pay.