Can I have my ex sign a paper in front of a notary allowing me to claim our youngest child on my taxes if I'm the custodial parent?

If you are the custodial parent, by definition you must claim you son as a dependent in your tax return. If you ex wants to claim him or a judge orders you to release the claim, you must sign a IRS form 8332 to release the dependency so that your ex can claim him. This must be done on a yearly basis.