When you transferred an interest in the property to your son, you made a completed gift. To the extent the fair market value of the gift exceeded $11,000, you have incurred both a gift tax liability as well as created a filing requirement.
The gift is reported on form 709 ( http://www.irs.gov/pub/irs-pdf/f709.pdf) where both your gift tax liability and exemption is computed. You receive an lifetime exemption credit equivalent to $1,000,000. To the extent your gifts to one individual exceeds $11,000 in one year, you will use up a portion of that exemption equivalent amount. Your son assumes your cost basis and holding period for the portion of the interest you gifted to him.
Based on the information you presented, it appears you gifted a 50% interest in the property to him. Accordingly, to the extent the fair market value of that interest (i.e. 50% of the FMV of the property on the date of gift) exceeds $11,000, you have made a taxable gift and should file a gift tax return. Its better to file late then not at all. You stand a better chance of avoiding penalties. You should get a full appraisal as of the date of the gift to support your valuation.
Please note, if your son gifts the property back to you, he will be in the same predicament.
Because it is impossible for me to identify and consider ALL the relevant facts, this advice is not intended or written to be used for the purpose of avoiding penalties, and cannot be used for that purpose.