What kind of form do I need to fill out if someone claimed my son and we had no agreement prior to doing so

There is no form or other way to report that.  You just file your own tax return and the IRS will sort it out.  See details below.

A child can be the “qualifying child” dependent of any close relative in the household.  The family  may come out better if grandma claims the child.  The smart thing to do is prepare returns both ways and see which way is best. Or you could Try this comparison  tool   https://turbotax.intuit.com/tax-tools/calculators/taxcaster/?s=1

There is an additional rule: in order for grandma (or any non parent relative) to claim the child, she must have a higher income than the parent.

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If someone else claimed your child inappropriately, and if they file first, your return will be rejected if e-filed. You would then need to file a return on paper, claiming the child as  appropriate. The IRS will process your return and send you your refund, in the normal time. Shortly (up to a year) thereafter, you'll receive a letter from the IRS, stating that your child was claimed on another return. It will tell you that if you made a mistake to file an amended return and if you didn't make a mistake to do nothing. The other party will get the same letter you did. If one of you doesn't file an amended return, unclaiming the child, the next letter, from the IRS, will require you to provide proof. Be sure to reply in a timely manner.

Winner gets the tax benefits; loser gets to pay the IRS back with penalties and interest.  The  parent almost always wins. Other relatives  can only claim the child as a dependent if the custodial parent gives permission or doesn't qualify to file.  The amount of support provided does not usually matter.

References:

https://www.thebalance.com/claiming-same-dependent-audit-risk-3193030 

http://taxes.about.com/od/dependents/qt/Dependents-Audits.htm

www.eitc.irs.gov/EITCCentral/f886-h-dep.pdf