From what Allfiredup posted, step-grandfather only passed away 2 months ago and probate has not been started yet.
Should i be concerned that his son might try to contest the will?
Have no idea if you should be concerned. As an heir (not the same thing as a beneficiary under the will), he has the right to contest the will if he desires to.
If he did contest it, what are the chances he would be awarded anything?
Nobody can answer that. He could argue that he was simply forgotten about/overlooked. The estate's personal representive would contend that, given the rocky history, he was certainly not just overlooked, that the will explicitly left all assets to the spouse, and that no other children, estranged or not, were included as beneficiaries.
Does the will contain a clause stating anything to the effect of: any person not mentioned in the will is not excluded due to oversight or neglect but because the maker of the will only wished to benefit the beneficiaries explicitly named in the will?
If it is any help, here is a website with information about probate in Georgia:
http://www.gaprobate.org/