OK.. I've had time to review information, and I have the answer (sort of) as well as some questions for you.
The SVT in and of itself is disqualifying, unless there has been no recurrence during the preceding 2 years while off all medications.
The fact that your son had an ablation done for SVT is a little suprising for me, as I was only aware of ablations being done for Wolff-Parkinson-White syndrome (WPW) or other ventricular conduction disorders. Granted medicine moves at the speed of light sometimes, and I haven't been keeping up recently.
So my question for you, I'm sure you are correct that your son had SVT, but is it possible that it might have been caused by a ventricular conduction disorder such as WPW?
Either way, as long as your son is asymptomatic for 2 years, and off medications, there should be no issues for him. If it has been less than 2 years, he will be found disqualified, but as long as he's been asymptomatic the chances of a waiver are good.