Can my son go to his fathers house? It has been 3 weeks since he was diagnosed with chickenpox they have scabbed and he has some scars from the ones he scratched open.

Once . Once the lesion have crusted over ("scabbed") then he is no longer contagious. The scars may not go away completely, they will fade over time. If this is an issue you can try over the counter mederma to reduce the appearance of the scars. Good luck.
It's . It's unlikely your son will present a threat to his father for two principal reasons: 1) his father has almost assuredly had chickenpox -- or been vaccinated against it -- at some time during his life, so he will be immune 2) new lesions cease to appear on the skin of infected individuals by the end of the first week, and the level of contagion drops significantly by the time the last pock has crusted over (usually by 10 to 14 days). By the end of the third week, the risk of your son transmitting chickenpox to anyone is essentially nil. I hope that helps.