No, you don't need a Realtor involved in this. If your son is getting financed, chances are he will want or need to go through the regular steps of having inspections and an appraisal done. His lender will be a good source of information as well. I would definitely still plan on having everything in writing.
Here's a link to the Oklahoma Real Estate Commission's purchase contract: http://www.ok.gov/OREC/documents/Residential%20Sales%20(11-2012R).pdf
If you are both a little contract shy, you could consider paying to have a Realtor prepare this for you and discuss the necessary steps to take between contract and closing, or you might consider a real estate attorney.
Best of luck to both of you and feel free to give me a shout if need a little guidance.
Metro First Realty West
1. Making sure the title process is initiated and followed through.
2. If ALL of the necessary documents aren't used, then you may open yourself, or your son up to legal difficulties in the future. You wouldn't want someone to contest the transfer some day after you have passed on, or in other situations.
There are other reasons, but these are the most important. I hope that helps!
Email me at Josh@JoshBarnettOKC.com or call me at 405-315-7965 and I can get your both started.