As far as I can tell from my tortuous communications with the CAA medical department, it will not issue an unrestricted Class 1 medical certification if the number of ectopic beats exceeds 2% (12 every 10 minutes at a heart rate of 60bpm).
I know this because I have just had an OML slapped on me because the percentage of ectopic beats I am getting is 10% along with couplets and triplets. This is in spite of having had an echo and stress ECG which show no sign of ischemic heart disease. Interestingly, I was allowed to hold an unrestricted Class 1 for over 8 months after discovery of the ectopic beats but now it has been shown I do not have heart disease, I have had a multi-crew OML placed on me
! Furthermore, the AMEs at CAA admit that there is no evidence of the prognostic significance of extra beats in otherwise normal individuals; so, it appears to me to be an a**e covering exercise!
I will be fighting this decision as I am unable to see the logic being applied,(the CAA hasn't followed the JAA ectopy protocol is dealing with my case!), and I suggest you do the same for your son. You might also want to investigate the possibility of your son going down the multi-crew licence path as it may be that he could still do this with a restriction to his Class 1. Good luck and keep updating the forum as I would be interested to hear the outcome.